I've found there are certain skills which are not normally stocked. These are like "secret" skills, and while you may find a scroll, you don't find the books unless it's a special chest or quest area (I found a secret place late in the game where you are presented with three skills which are supposedly rare but you can only choose one to take home). Summon skeleton warrior and summon wolf fell into these categories for me. I had to craft both of these books myself to get them. Eventually I found a copy of summon skeleton warrior in the Undertaker's hut in Cyseal, sitting on a table. I have never seen a book of it anywhere else, though I have seen scrolls. For summon wolf I've only seen a handful of scrolls and never a book (and I'm level 17 atm).
I believe Rain of Arrows, Icewall and Lightning bolt are in this category too, although after I found the Rain of Arrows book in the secret library it started showing up in the earth elemental's inventory for me at the end of time. I haven't a book or scroll of lightning bolt yet, though I've collected three icewalls, all of which I made into books.
Regarding re-stocking, I've only played co-op, and I've found it's best to save before talking to a vendor and then reload if they don't sell what you want, especially if you suspect a refresh is about to happen. Personally, I think it should happen more often, and they should say something like "Got some new items in stock" or whatever so you know you're not dreaming.
