I don't use Merge because of Find Duplicates.
It's because I discover, usually via an external content search, that the same text was published under two different titles in different publications.
I mark the two books, show Marked books, sort list so the book I want to remain in place (target) is topmost, select using ctrl+A and do the merge.
Trying to control which book is the target via the order of selection has never worked for me - maybe it does if you use mickey mouse clickety clicks
BR