Going through the list of stuff in MY database that is not in YOUR database. Item in question is listed as "X-men: The Ultimate Guide", a book by DK in my comic list. Now, the actual name of this book is "Ultimate X-Men". When I try to add it the books section, I get this pop-up: "Had trouble saving this issue: constraint failed FOREIGN KEY constraint failed".

Any advice?

The pop-up window title is "Can't save this issue"

This doesn't answer your question directly, but...

If you have a Title in Comic Books and it needs to be moved to Books, you can use menu item Edit->Change Title Media Type to let the system move it for you.

Also, you might try File Tools-> Optimize Database and see if that clears up any problems.

13 hours ago, Michael E. Jacobson said:

Going through the list of stuff in MY database that is not in YOUR database. Item in question is listed as "X-men: The Ultimate Guide", a book by DK in my comic list. Now, the actual name of this book is "Ultimate X-Men". When I try to add it the books section, I get this pop-up: "Had trouble saving this issue: constraint failed FOREIGN KEY constraint failed".

Any advice?

The title name may already exist. You might have to mod it to Ultimate X-Men (DK)