Skills are not gear.
By allowing skillbooks to be random for such a limited selection of skills, it doesn't improve gameplay but detracts from it. If you create a character with the intention of being a fire mage - and suddenly find you cannot ever find fire elemental, you're incredibly weak (but playable).
If you plan the same character as a lightning mage but cannot obtain blitz bolt... you're useless.
The random attitude of vendors concerning skillbooks is a poor one simply because there are only what, 4 spells per level? So it's not like a DnD Wizard who cannot learn fireball because he'd have acid splash, lightning bolt and cone of cold (sure, CoC is level 4 not 3 but you get the point). This game's spell list does not create alternatives within a specific tree. Without key spells you are so weak you're unable to play what you intended to.
Were there more skills per level, with choices (imagine if there were 3 1 turn cd fire spells spread between levels 1 and 4 giving options... you'd only need to find 1 of them).
So yeah, I can agree it'd be nice to have some random skill find, except skills are not treasure... skills are no different to talent points. Being unable to choose them is not fun, regardless of whether it's fun to discover them in the world.
