I made note of all the spellbooks that the merchants had, and never chose those spells when leveling. As a result, a few times during the game my spellcasters had no spell picks to make, which made the process of waiting a lengthy time to use the free spell slots less annoying.
I used Myles to grab the spellbooks. Why not increase his pickpocketing? I consider it a tertiary skill, and as he's not a permanant member of my party, nothing's lost by putting so many skill points into it. (My own party philosophy includes going for all the profession's important combat-related skills; my bard grabs all the music she can; and later in the game, everybody goes after artifacts. I want a party that can sling powders and bombs with the best of 'em.)
Spellbooks are level 1-4, which doesn't mean you'll find all lower level spells in books. There's a certain random factor involved in what shows up and where, but I've yet to see a book vanish from a merchant if you don't grab it. Instead, the stock seems (in my experience) to remain stationary. Once you buy something that lessens a full inventory, new, sometimes different items are added. You'll also find books, frequently in library-style rooms, occasionally in odd areas where you wouldn't think to look. I strongly urge adding a ranger to a party, since it increases the likelihood of quickly locating hidden books. The Cast Elemental spell is a joy: even at a low level, it gives you a target to draw fire. And don't forget the spells that mess with your opponents' minds! Even if you like Fireball and such, spells like Insanity, Blindness, and Terror are still truer friends. They can turn invasions and into routs.