I notice that this achievement is the only one marked Legendary and has no percentage at all. I assume no one has gotten this achievement and I think it is because it is bugged.

I'm certain I have read everything.

Yes it is currently bugged. These days I have been playing the 3rd game and read all books but achievement didn't unlock.

All 3 trails in the sky games seems to have achievement problems...(Also trails of cold Steel 2 seems to have problems with unlocking achievements. but there is a way to revert to a previous version fo Trails of cold steel 2, That is if you ever play TOCS II
https://www.gog.com/forum/the_legend_of_heroes_trails_of_cold_steel_series/achievements_have_stopped_popping_did_i_disable_them_somehow)

Maybe there are issues but I was able to get 100% on FC and SC. And all of them for The 3rd EXCEPT bookworm.

I see this is what the latest update fixes, after you asked about it on steam :/.

https://steamcommunity.com/app/436670/discussions/0/3069740688715607845/

Yes it is now working.