I'm only lvl 26 so far and I dunno if this has been mentioned before or goes without saying, but just as a tip:
- If you were unable to buy certain points from trainers when you first met them, when and if you reset your destiny you can go back and buy those extra points. When you reset your destiny, you only get refunded the points you've acquired via lvl'n and obviously NOT book or bought SP
So far, I've only met 2 trainers for some skills (eg. there's a Detect hidden trainer in Gohart and one in Ysa, BS trainers at Shieldring Keep and Ysa and a few others). You can't train more than once at the same trainer but you can per skill
Also, I'm unsure how many trainers there are per skill and they can train you up to lvl 4 of a skill (will only train you if you have 0 - 3 SP already), so if there are 4 trainers, it would be wise to keep the SP books you've found until you've bought all 4 points first.
Someone who's completed can let us know if there are 4 trainers per skill or not. If not, then I guess it doesn't matter when you use the books. There's also your starting points to take into account. So if you found a book for skill you started with some points in, hold onto it.
If this is incorrect or too basic too mention or even if it is explained somewhere game, please rectify me. It just may help someone else.