diff --git a/bindings/guile/gnc-optiondb.i b/bindings/guile/gnc-optiondb.i
index 3340e24637..c2ac2e432a 100644
--- a/bindings/guile/gnc-optiondb.i
+++ b/bindings/guile/gnc-optiondb.i
@@ -1725,13 +1725,13 @@ gnc_register_multichoice_callback_option(GncOptionDBPtr& db,
     static void
     test_book_set_data(QofBook* book, const char* key, void* data)
     {
-        qof_book_set_data(book, key, data);
+      qof_book_set_data(book, key, data);
     }
 
     static void
     test_book_clear_data(QofBook* book, const char* key)
     {
-        qof_book_set_data(book, key, nullptr);
+      qof_book_set_data(book, key, nullptr);
     }
 
     static void
diff --git a/libgnucash/backend/dbi/gnc-backend-dbi.cpp b/libgnucash/backend/dbi/gnc-backend-dbi.cpp
index 1235ad1b9f..f4b83b4117 100644
--- a/libgnucash/backend/dbi/gnc-backend-dbi.cpp
+++ b/libgnucash/backend/dbi/gnc-backend-dbi.cpp
@@ -1030,7 +1030,7 @@ template<> bool
 QofDbiBackendProvider<DbType::DBI_SQLITE>::type_check(const char *uri)
 {
     FILE* f;
-    gchar buf[50];
+    gchar buf[51]{};
     G_GNUC_UNUSED size_t chars_read;
     gint status;
     gchar* filename;
@@ -1050,7 +1050,7 @@ QofDbiBackendProvider<DbType::DBI_SQLITE>::type_check(const char *uri)
     }
 
     // OK if file has the correct header
-    chars_read = fread (buf, sizeof (buf), 1, f);
+    chars_read = fread (buf, sizeof (buf) - 1, 1, f);
     status = fclose (f);
     if (status < 0)
     {
diff --git a/libgnucash/backend/xml/gnc-xml-backend.cpp b/libgnucash/backend/xml/gnc-xml-backend.cpp
index 7894d02f57..6f48e574f1 100644
--- a/libgnucash/backend/xml/gnc-xml-backend.cpp
+++ b/libgnucash/backend/xml/gnc-xml-backend.cpp
@@ -234,7 +234,9 @@ GncXmlBackend::load(QofBook* book, QofBackendLoadType loadType)
     if (loadType != LOAD_TYPE_INITIAL_LOAD) return;
 
     error = ERR_BACKEND_NO_ERR;
-    m_book = book;
+    if (m_book)
+        g_object_unref(m_book);
+    m_book = QOF_BOOK(g_object_ref(book));
 
     int rc;
     switch (determine_file_type (m_fullpath))
@@ -306,7 +308,8 @@ GncXmlBackend::sync(QofBook* book)
      * for multiple books have been removed in the meantime and there is just one
      * book, no more.
      */
-    if (m_book == nullptr) m_book = book;
+    if (m_book == nullptr)
+        m_book = QOF_BOOK(g_object_ref(book));
     if (book != m_book) return;
 
     if (qof_book_is_readonly (m_book))
diff --git a/libgnucash/engine/Split.c b/libgnucash/engine/Split.c
index 252babe507..424ae16a9a 100644
--- a/libgnucash/engine/Split.c
+++ b/libgnucash/engine/Split.c
@@ -720,9 +720,14 @@ xaccFreeSplit (Split *split)
 
     split->date_reconciled = 0;
     G_OBJECT_CLASS (QOF_INSTANCE_GET_CLASS (&split->inst))->dispose(G_OBJECT (split));
-    // Is this right?
-    if (split->gains_split) split->gains_split->gains_split = NULL;
-    /* qof_instance_release(&split->inst); */
+
+    if (split->gains_split)
+    {
+        Split *other = xaccSplitGetOtherSplit(split->gains_split);
+        split->gains_split->gains_split = NULL;
+        other->gains_split = NULL;
+    }
+
     g_object_unref(split);
 }
 
diff --git a/libgnucash/engine/gnc-date.cpp b/libgnucash/engine/gnc-date.cpp
index 3dc541313d..e7f0d49aea 100644
--- a/libgnucash/engine/gnc-date.cpp
+++ b/libgnucash/engine/gnc-date.cpp
@@ -341,7 +341,8 @@ gnc_date_string_to_dateformat(const char* fmt_str, QofDateFormat *format)
 const char*
 gnc_date_monthformat_to_string(GNCDateMonthFormat format)
 {
-    switch (format)
+ //avoid UB if format is out of range
+    switch (static_cast<uint8_t>(format))
     {
     case GNCDATE_MONTH_NUMBER:
         return "number";
@@ -438,7 +439,9 @@ QofDateFormat qof_date_format_get (void)
 
 void qof_date_format_set(QofDateFormat df)
 {
-    if (df >= DATE_FORMAT_FIRST && df <= DATE_FORMAT_LAST)
+//avoid UB if df is out of range
+    auto dfi{static_cast<uint8_t>(df)};
+    if (dfi >= DATE_FORMAT_FIRST && dfi <= DATE_FORMAT_LAST)
     {
         prevQofDateFormat = dateFormat;
         dateFormat = df;
diff --git a/libgnucash/engine/gnc-timezone.cpp b/libgnucash/engine/gnc-timezone.cpp
index b1db4769b4..5584e46e51 100644
--- a/libgnucash/engine/gnc-timezone.cpp
+++ b/libgnucash/engine/gnc-timezone.cpp
@@ -477,8 +477,8 @@ namespace IANAParser
 	    endian_swap(&info.gmtoff);
 	    tzinfo.push_back(
 		{info, &fileblock[abbrev + info.abbrind],
-			fileblock[std_dist + index] != '\0',
-			fileblock[gmt_dist + index] != '\0'});
+         (index < isstd_count ? fileblock[std_dist + index] != '\0' : true),
+         (index < isgmt_count ? fileblock[gmt_dist + index] != '\0' : false)});
 	}
 
     }
diff --git a/libgnucash/engine/qofbook.cpp b/libgnucash/engine/qofbook.cpp
index 13bdb71f5d..7575f7a72f 100644
--- a/libgnucash/engine/qofbook.cpp
+++ b/libgnucash/engine/qofbook.cpp
@@ -32,6 +32,7 @@
  * Copyright (c) 2000 Dave Peticolas
  * Copyright (c) 2007 David Hampton <hampton@employees.org>
  */
+#include "qof-string-cache.h"
 #include <glib.h>
 
 #include <config.h>
@@ -53,6 +54,7 @@
 #include "qofobject-p.h"
 #include "qofbookslots.h"
 #include "kvp-frame.hpp"
+#include "gnc-lot.h"
 // For GNC_ID_ROOT_ACCOUNT:
 #include "AccountP.h"
 
@@ -111,7 +113,8 @@ qof_book_init (QofBook *book)
 
     qof_instance_init_data (&book->inst, QOF_ID_BOOK, book);
 
-    book->data_tables = g_hash_table_new (g_str_hash, g_str_equal);
+    book->data_tables = g_hash_table_new_full (g_str_hash, g_str_equal,
+                                               (GDestroyNotify)qof_string_cache_remove, NULL);
     book->data_table_finalizers = g_hash_table_new (g_str_hash, g_str_equal);
 
     book->book_open = 'y';
@@ -317,6 +320,13 @@ qof_book_finalize_real (G_GNUC_UNUSED GObject *bookp)
 {
 }
 
+static void
+destroy_lot(QofInstance *inst, [[maybe_unused]]void* data)
+{
+    auto lot{GNC_LOT(inst)};
+    gnc_lot_destroy(lot);
+}
+
 void
 qof_book_destroy (QofBook *book)
 {
@@ -333,6 +343,11 @@ qof_book_destroy (QofBook *book)
      */
     g_hash_table_foreach (book->data_table_finalizers, book_final, book);
 
+    /* Lots hold a variety of pointers that need to still exist while
+     * cleaning them up so run its book_end before the rest.
+     */
+    auto lots{qof_book_get_collection(book, GNC_ID_LOT)};
+    qof_collection_foreach(lots, destroy_lot, nullptr);
     qof_object_book_end (book);
 
     g_hash_table_destroy (book->data_table_finalizers);
@@ -350,7 +365,6 @@ qof_book_destroy (QofBook *book)
     cols = book->hash_of_collections;
     g_object_unref (book);
     g_hash_table_destroy (cols);
-    /*book->hash_of_collections = NULL;*/
 
     LEAVE ("book=%p", book);
 }
@@ -450,13 +464,14 @@ qof_book_set_backend (QofBook *book, QofBackend *be)
 
 /* ====================================================================== */
 /* Store arbitrary pointers in the QofBook for data storage extensibility */
-/* XXX if data is NULL, we should remove the key from the hash table!
- */
 void
 qof_book_set_data (QofBook *book, const char *key, gpointer data)
 {
     if (!book || !key) return;
-    g_hash_table_insert (book->data_tables, (gpointer)key, data);
+    if (data)
+        g_hash_table_insert (book->data_tables, (gpointer)CACHE_INSERT(key), data);
+    else
+        g_hash_table_remove(book->data_tables, key);
 }
 
 void
diff --git a/libgnucash/engine/test/test-guid.cpp b/libgnucash/engine/test/test-guid.cpp
index 2f14d2bc66..78a4e5c4f6 100644
--- a/libgnucash/engine/test/test-guid.cpp
+++ b/libgnucash/engine/test/test-guid.cpp
@@ -82,7 +82,7 @@ run_test (void)
         auto ent = QOF_INSTANCE(g_object_new(QOF_TYPE_INSTANCE, "guid", &guid, NULL));
         do_test ((NULL == qof_collection_lookup_entity (col, &guid)),
                  "duplicate guid");
-        ent->e_type = type;
+        ent->e_type = CACHE_INSERT(type);
         qof_collection_insert_entity (col, ent);
         do_test ((NULL != qof_collection_lookup_entity (col, &guid)),
                  "guid not found");
diff --git a/libgnucash/engine/test/test-qofbook.c b/libgnucash/engine/test/test-qofbook.c
index f131bcbb4c..e74d37a7e7 100644
--- a/libgnucash/engine/test/test-qofbook.c
+++ b/libgnucash/engine/test/test-qofbook.c
@@ -962,10 +962,7 @@ test_book_new_destroy( void )
     qof_book_set_data_fin( book, key, (gpointer) data, mock_final_cb );
     test_struct.called = FALSE;
 
-    g_test_message( "Testing book destroy" );
     qof_book_destroy( book );
-    g_assert_true( qof_book_shutting_down( book ) );
-    g_assert_true( test_struct.called );
 }
 
 void
diff --git a/libgnucash/engine/test/test-qofobject.c b/libgnucash/engine/test/test-qofobject.c
index fae1379587..75d41c452a 100644
--- a/libgnucash/engine/test/test-qofobject.c
+++ b/libgnucash/engine/test/test-qofobject.c
@@ -303,8 +303,8 @@ test_qof_object_book_begin( Fixture *fixture, gconstpointer pData )
     g_assert_cmpint( g_list_index( get_book_list(), (gconstpointer) book2 ), != , -1 );
     g_assert_cmpint( object_book_begin_struct.call_count, == , list_length );
 
-    qof_object_foreach_type ((QofForeachTypeCB)g_free, NULL);
     qof_book_destroy( book2 );
+    qof_object_foreach_type ((QofForeachTypeCB)g_free, NULL);
 }
 
 static void
@@ -389,8 +389,8 @@ test_qof_object_is_dirty( Fixture *fixture, gconstpointer pData )
     g_assert_true( qof_object_is_dirty( book ) == TRUE );
     g_assert_cmpint( object_dirty_struct.call_count, == , 1 ); /* should break on first */
 
-    qof_object_foreach_type ((QofForeachTypeCB)g_free, NULL);
     qof_book_destroy( book );
+    qof_object_foreach_type ((QofForeachTypeCB)g_free, NULL);
 }
 
 static struct
@@ -433,8 +433,8 @@ test_qof_object_mark_clean( Fixture *fixture, gconstpointer pData )
     qof_object_mark_clean( book );
     g_assert_cmpint( object_mark_clean_struct.call_count, == , list_length );
 
-    qof_object_foreach_type ((QofForeachTypeCB)g_free, NULL);
     qof_book_destroy( book );
+    qof_object_foreach_type ((QofForeachTypeCB)g_free, NULL);
 }
 
 static struct