Theologia Arcanea (book of divine magic) - Bought at church

Adversum Maleficarum (book of magic of true faith) - Bought at church

Diquisitiones Magicae (book of magic of miracles) - Bought at church

Directorium Inquisitorum (book of inquisitorial magic) - Bought at church

Liber Artum Magicarum Prohibitarum (book of heretical magic) - ?

Elenchós Tés Pseudonymú Gnóseós (book of pagan magic) - Found in a chest in the mine. It's very deep into the mine (Orcs Cave Level 3, NOT Iron Mine Level 3) and in the same room as a portal.

Liber Lucifer Ordinis Novi (book of infernal magic) - Found in a chest outside the mine in a orc camp

I'm pretty sure all books can be bought. Including the book of heretical magic. Funny how you sometimes see those in the inventory of priests...

I have yet to see any forbidden magic books for sale. It's possible that they will be available later in the game but you can find them faster if you explore which was my goal with this topic. Though I have yet to find a Book of Heretical magic.

I'm definitely sure you can buy them, I remember the 1st time I saw one for sale I was like "wtf it's supposed to be forbidden". The seller's inventory is randomized each time you speak to them so it might take a few tries to see the books?

Well, at the very least they're definitely not available from Dominicus in Hillbrandt. I've been to his store many times and he always has the same four books, in the same order, in the same place in his inventory. Definitely not randomized. I assume there're more spell merchants later on though, right?

Maybe it only randomizes scrolls, but 100% sure you'll find forbidden books for sale as well. And yes there are more merchants. The herbowomen and alchemists may have some books too (man I just finished this game like 2 weeks ago and already forgot half of it).

I just got to act 2 and the first merchant I encountered sold the Pagan book. That said I don't think it is completely randomized since no merchant in act 1 sold any forbidden magic books. Also, I have yet to find the Heretical Magic Book. I assume I can buy it from a merchant now but is it possible to get it at act 1?

You can buy ALL books from the genies shop. To get a genie break open one of the small green magic cubes. This generates a random inventory. If the item does not appear. Reload and rinse repeat eventually it will appear in the inventory of the genie shop. Bear in mind you can break a small green cube anywhere spawning the genie and then shop.