The History of Asmodeus
So you'd like to learn more about Asmodeus, but you don't want to buy commercial books that are mostly just excerpting other sources and padding things out with UPG? Understandable! Here's a reading list based on the sources provided in one of the more popular books about Asmodeus currently on the market.
I don't work with Asmodeus and I put this together rather quickly, as a starting point for research, not a comprehensive accounting. I am not particularly knowledgeable or well-read in Zoroastrianism, and am using Peterson's site for sourcing. Feedback on this section is welcome. I will also note that the better-known Asmodeus book suggests a syncretic connection between Aeshma and Kartikeya, and recounts some Hindu mythology in order to make that argument. I have skipped over that material and added a few other citations that I think are directly informative.
First, here are the Zoroastrian sources:
Yasna
Yashts 10, 11, 19
Vendidad
Shahnameh by Ferdowsi
Greater Bundahishn
Zand-i Vohuman Yasht
Next, the Jewish sources:
The Book of Tobit
Treatise on the Left Emanation by Isaac ben Jacob Ha-Kohen
The Book of Legends: Tales from the Talmud and Midrash by Hayim N. Bialik and Yehoshua H. Rawnitzki
From there, Asmodeus enters the grimoire tradition:
The Testament of Solomon
Liber Bileth
Liber Juratus
Hygromanteia
The Book of Abramelin
Three Books of Occult Philosophy by Heinrich Cornelius Agrippa
Pseudomonarchia Daemonum by Johann Weyer
Ars Goetia
Dictionnaire Infernal by Jacques Collin de Plancy
Compendium Rarissimum
The Magus by Francis Barrett
These modern publications of Faustian and other magical texts include some references to Asmodeus:
The Book of Oberon by Daniel Harms, James R. Clark, and Joseph H. Peterson
The Key of Necromancy by Nicolas Alvarez Ortiz
The Black Books of Elverum by Mary S. Rustad
Touch Me Not: A Most Rare Compendium of the Whole Magical Art by Hereward Tilton and Merlin Cox
Literary and other sources:
The legend of the Shamir
Lanterne of Light and other works related to the Seven Deadly Sins
Malleus Maleficarum
The Limping Devil by Luis Vélez de Guevara
The Devil on Two Sticks by Alain-René Lesage
The Letter, the Witch, and the Ring by John Bellairs
This should get you up to speed on his verifiable lore, and if anybody who works with him would like to contribute some additional suggestions in the comments, that would be appreciated.
The post image is from the first edition AD&D Monster Manual.