If you mean the levels to specific skills from specific lorebooks these are limited.
Once you have unlocked the eidetic memory by completing the mages guild skill line you can see all the books available in the game within the lore library. I think there are 64 skill books available in the game. They drop randomly from bookshelves and each only gives the level increase once.
Many people find once they have the eidetic memory unlocked that they start chasing all the books in the game, it's just something else to complete.
I just read them as leveling if they're in content I'm doing. If I get them I get them.
Once your skill line is at 50 they don't matter anymore.
They're a remnant of when leveling in this game took significantly longer as a means for a quick "boost"
Thanks to both of you.
They do give a single point for a companion, so some merit every so often (5 minutes?) but good to know not to worry once I am at max. I have found it quite inconsistent across my alts.