The book skill points are an instant skill level up rather than an xp boost. And the books in the other alliance zones are likely (for the most part) the same ones you found while leveling 1-50 since VR zones are just the scaled up versions of the low level zones and they would have made sure each of the alliances had the same skill books, albeit in differrnt locations. Any new skill books you find in vet zones are probably ones you missed while leveling. Just my theory.

From my experience:

There are a finite number of books of several types. Zone Lore, Cultural Lore, Skill (Crafting, Class, Weapon, etc.) Pamphlets, papers, & scrolls, etc. that are location specific, like NPC journals and such appear in areas for flavor too.

Certain books, like the Zone Lore subjects appear in the appropriate zones. you get a message when you find, for example, all 20/20 'Some zone' Lore books.

Once in VR area, you may notice many of the book locations, where you know there is a book, appear empty, since you already found that book in a previous location/zone.

There are a finite number of crafting, class, and weapons skill books that you can find. Once you have found the 5th(Not sure what the limit is, but I am feeling like there is one?) +Aedric Spear class skill or +Alchemy book, you'll never find another.

Bookshelves contain a completely random book, or no book, and can contain a different book the next time the zone loads for you.

I am currently conducting an experiment with a new lvl 20 char that has only, ever, read one bookshelf (mis-clicked). I plan to get him to lvl 45+ or have a lot of crafting skills at 45+ before I start reading random books from bookshelves.

Now do keep in mind this is just my observation, without real study or record keeping, but from 7 played chars, all 19+ lvl, one at VR3.

I am currently doing the same.

I noticed, on my first character, that I stopped getting skill points from books sometime around level 40. And getting those skill points to level one of your main skill trees from 5 to 6 felt like a waste.

So, on my new character, I decided to hold off on reading until I got him to level 40. I've been getting skill points from a lot of books since then, and it's definitely more pleasing to get a skill from 48 to 49 by reading.

Level 40 feels like a good moment to start reading, as you're getting near the 50 points in your main skill trees. I went back to all the Mages Guild halls, and a few Ayleid ruins with a bunch of books, and managed to level a few of my skill trees to 50 by doing this.

the bookshelves always have randomized books. I did the ebon pact questlines a few times over now, and have managed to memorize certain bookshelves that increased a certain skill, well I go to this bookshelf, and instead of lvling up my bow skill, like it did my last play thru it lvled up something else. if you checked every single bookshelf in your home alliance, im sure you wont find any skill books in the vet zones, I have yet to find any skill books in the vet zones. they likely, as someone above posted, put the same books in each alliance, though in different shelves. so if you went thru every shelf in your home alliance, I doubt you will find any skill books In the vet zones

I believe that all the bookshelfs in VR zones are pretty much a mirror copy of what you have in your own faction zones. So you basicaly read same books which already agve you a skill increase.

I wonder if any try to get to VR zone with out openening any bookshelfs and then open them in VR zone.