Firstly things seem a lot more stable with 6.14.
I did get a database out of range error when adding a new book by drag 'n' drop, checking db integrity produced no errors but shorly afterwards I got this error and ctd.
Code:
ASSERT failure in QCoreApplication::sendEvent: "Cannot send events to objects owned by a different thread. Current thread 5a8fb30. Receiver '' (of type 'QWidget') was created in thread 39120d0", file kernel/qcoreapplication.cpp, line 305
The book added when the db range error occurred seems to be intact and I was searching for another book at the time of the fatal crash.