When you trigger a book shelf, they can give exp towards a set skill. If you do this early, it will level that skill, if you do it later, you will still get the same exp, but it will not be enough to level the skill. So it makes no difference as to when you trigger it.

Its just hard to 'track' the skill based exp without add-ons, but they are the same amount. (But its capped, it will never be more then one skill up for a set skill from a bookshelf)

So for the best value, get most skills to around ten, so you get the most out of the bookshelf 'exp' but there are SO many bookshelf, its not worth worrying about it.

Later on you will click 30+ in a row and think your getting nothing, but you are (if you have an add-on to track it) its just not enough to level it.

By the time your in your Veteran levels, you will see its too easy to level most things to even worry about clicking on bookshelf's anymore

its not true
you always will get EXP for WHOLE LEVEL