Please be aware that some significant layout-altering bugs that have been around for quite a few months, were at least partially fixed in 2.3.2.
Therefore, if you have been using the option to “Start new book on same page…” option, then updating to 2.3.2 will: (1) replace the current double line between books with a single one.
(2) by not having the double line, reduce the gap between the old book and the new one
(3) Therefore, probably fit more of the book on that page.
(4) Therefore, result in different page breaks. This may be just what you want, (or it might be a complete pain if you’re part-way through fine-adjusting something).
ALSO it should avoid splitting the book title from any following body text.
If you need the double-line between books, then it should be possible using hooks.