If you like crafting, another way to get summons is to make them.

Finding the scrolls is easier than finding books for a few summons in particular (notably: summon wolf, and I don't know about you but my witch started with a summon skeleton scroll). Then you can put the scroll in a blank book of the same type to make them. If you really gun for it, around level 7 or so you can get to 5 crafting if you take the scientist talent and make an effort to find both a belt and a bracer with + crafting on them.

We did it on our 2nd play-through, in time to make the skeleton scroll our witch started with into a book and learn it. The crafting guide will cover how to make the raw materials, and with recent patches we found a lot of blank pages (and even some blank books) I don't remember seeing on our first play-through.