Most of the ships had libraries and one library had a dedicated book exchange cabinet. I simply released books there.
The one ship that didn't have a dedicated library had a book trolley that was brought to a certain lounge at pre-set times. The crew member that played ship librarian (eg took note of which passenger borrowed which ship-owned book) was happy for me to "donate" my own books and put them on the bottom of the trolley.
On each cruise, I noticed several passengers wandering around with books, especially on sea days. I'd see people reading on deckchairs out by the pool or in a quiet lounge. And at the end of each cruise, there always seemed to be a sudden rush of books left in the library on the last day!
I didn't bother releasing books elsewhere around the ship as it looked as if they'd be cleared away pretty quickly by crew members.
Example of a few releases:
PS Releases can also be made on shore excursions too!