Since I think most of us want the best of both diablos bundled into Diablo3 I would certainly like to Diablo2's skill system to be back but I would welcome a fourth tab with "acquired skills" where you could once again learn skills (or maybe even passives) from books. Just like you could in Diablo 1.
I dont have too much insight on how the skills would work, but I would suggest something like this:
*Acquired skills lvl up everytime you read a book, just like Diablo1.
*But you could lvl up any skill learned from books until lv 5.(which would be a big selection of skills, like Diablo1), so you could identify which skills you would find useful.
*You could choose 10 skills to be specialized. Those specialized ones could be lvled past lv 5 until lv 20.
*Requirements to lvl up those skills would include "Magic" (or energy, whatever it will be called) for spells or "Strenght" "Dexterity" for passive skills, player lvl, maybe even some achievements like "Defeat Diablo on Hell difficult" or "Find The *Unique*", and so on and on.
Whatever, I just would love to see a mix of Diablo1's book skills and Diablo2's character skills. I like how Diablo2's has more specific skills on the classes but I would like to see the books return as well, I am sure if done right It could be a lot of fun because it would diversify the skills choices a lot.
And of course that those skills would be mostly "support skills" (like heal), it wouldnt be quite possible to built your character based on those skills (the class skills have that function) but it would be that trumph card or extra little trick that could help you in times of need.
Anything that makes Energy more useful (beyond your typical sorc builds) is something that makes the game better. as it is currently in D2, Blizzard could've taken the Energy stat out (leaving mana) and the game would've gotten better.
In D2, they added lev 3 firebolt, charged bolt, etc on items so anyone can use/proc them. However, in hell mode (where the game truely begins for most of us) these spells dont even tickle the bad guys. They do absolutely nothing. It seemed to be a complete waste of programmers effort/memory space, etc. That is what I am worried about with the book idea. However, as long as it is possible to make the spells get to useful levels, then maybe they could make it work.
The books added an intellectual aspect to D1 and I was sad to see D2 reduce the idea to the petty tp and id scrolls which they thought were the only useful remnants from D1.
Ah yes, skill usefulness at high levels is something that needs to be looked up. I am not asking to make every max lvl skill to be ultra-powerful but it gotta have some use on Hell difficult.
A passive that diablo2 desperately needed (that could be learned via books) was a passive that could increase the damage caused by those throwing potions (alchemy could be the name). Easily the most useless feature on Diablo2 but had potential to be a lot of fun.
Those book skills would be just that, skills that can somehow support certain extra tricks of your characters.
What I'd like to see from skills is the separate trees per class for the core skills, plus a fourth tree for secondary skills that all players would have access to.
You would get a skill point in this fourth tree every other level (or every fifth level or something) and they would be passive skills for non-combat related bonuses. One would increase your extra gold from monsters, one would increase your stash size, one would slightly increase your magic find, etc.
You could also use regular skill points in this tree if you've maxed out your build and feel you can spare them.
Maybe you could also get extra points for certain skills in this tree from books like you were saying, but they would be rank specific. For example, you could find a Level 3 Extra Gold book, so you wouldn't be able to use it to progress from level 1 to 2, or from 3 to 4. That way it would be harder to farm the books.