While stuck indoors due to dense fog on a serious character, I decided to start working on metalworking by barricading and unbarricading a window in my safehouse.
Fast Learner, 3x multiplier from reading a book, and barricading gave me 1.95xp. That would be the expected 1.5xp x 1.3 from Fast Learner with no book modifier.
Weird. I hear the occasional comment about book xp boosts bugging out, so figure this might be one of those cases. I load up a test character to check things out more.
2.7x multiplier from a book...and I get 1.5xp again.
I decide to try out plank barricades. This character has Handy, so they have a baseline +400% xp boost to carpentry. Sure enough, building a plank barricade gives 3xp (0.75 x 4).
I read a carpentry book a bit to get a 2x multiplier. Building a plank barricade still gives me 3xp.
So, the bonus xp from being proficient in a skill gets applied, but not the books.
I create a new test character to test this hypothesis further, this time a metalworker.
With the full bonus from starting with +3 metalworking, building a barricade gives me 9.96xp.
Reading a metalworking book to get a 0.5x multiplier, and a barricade still gives me 9.96xp.
Reading the book to get a 1.5x multiplier and, yet again, 9.96xp.
So, this is pretty clearly a bug with barricading. Deconstructing had its xp gains properly modified by the book multipliers.
The game is completely unmodded.