I flashed my JBL to release version 16 (not beta) obtained here
http://www.jetbook.net/update.php?refid=29532
I went to the middle of a couple of books, closed them, shut down and changed batteries, went back to same books and they were at page 1 (that's right, one). Checked settings, and 16 WAS installed.
Looks like 15G bug is back to me. Flashed back to 15D-B once again, and all works fine.
Could they have fixed this in 16 Beta, then reintroduced the same bug in 16 released?