I picked a dedicated librarian/mechanic in Moonhome; their Wordsmith skill rusted away, though was eventually renewed with dozen points to dabbling - from Krogoth the to-be omnilegendary thread, you apparently use that with poetry when performing or arranging forms, and poets can write books. This one didn't get the chance to use it, but seems all five dwarves I'm atm looking at who have written books have 12-24 exp in it, a number that doesn't correlate with the 50-200 exp in writer.
I'm not sure what the reader skill does in fort mode - beyond being gained alongside student when reading books (but unlike in adventure mode, don't need it to read one). I know that worldgen scholars come with skill levels in reader, writer and student, though.
Perhaps related was that I got great book on animal migration out of military person (so high master student) with no significant values (perseverance/decorum/dignity) I assigned to library as their need was in the red, who pondered up from nothing to still pretty low adequate tracker/logican/record keeper (and also had 1 better organizer, but that's probably demonstrations), while the designated library-dwarf never made anything good. Probably nothing, as even a dabbling dwarf can make an exceptional coffer, but...
@Sanctume:
I'm absolutely certain - as in, have game open right now - that writer skill is at the very least gained by scholars who write books sometimes and never gained for scribes, even after copying dozens of books.
From this I dare hazard a guess that it relates to the quality of the content in produced book, but I have no real way to check right now.
One way to test for skills would perhaps be finding a librarian writing a book in mid-job, saving the game and save-scumming to check the distribution of book qualities; then using make-legendary (or higher with gui/gm-unit) to increase desired skills to test for and see if the writing quality changes.