Well, if your looking for good pick enchants on Books beside Efficiency, I recommend you go caving like a mad man and raid one dungeon after the other.
Because, as mentioned Before, the Chance of getting Silk Touch on a book is less then 1% (although, if youre going the "enchant-many-books" way i recommend enchanting your Book at lvl 18 instead of lvl 30... the chances for Silk Touch is the same)
So, go and raid some Dungeons because the chance of finding a Silk Touch book in a dungeon chest is significantly higher than getting it by enchant.
I found around 15 spawners and got 1 Silk Touch, 2 Fire Aspect II and 3 Fortune III books out of the chests.