Submitted: Is it alright to ask what sources Tibetan gods come from?
I have to admit I spent a little time trying to figure out if this was a cosmological question (when were these gods born in their mythology, and how, and from what, and what does that mean for the world?) or a scholarly one (what books are you reading about these gods in)? This post is going for the second one, so if you're the submitter and I got it wrong, ping us again and let us know!
If I had to give Tibetan deities a seriousness rating, it would be: EXTREMELY
When we talk about the Tibetan gods, we're usually referring to the gods of Bön, the major religion native to Tibet. Of course, it's not even close to the only religion in Tibet; Tibetan Buddhism, imported from India and heavily influenced by China and Mongolia, is at least equal to Bön in terms of popularity in Hero's Journey, and there are at least three major strains of Bön itself in addition to folk religious beliefs that aren't as formalized but have nevertheless been around forever.
As you might imagine, it's not the easiest religion to find good information about in the West, in English, while China is still busy trying to demand Tibet stop having religion at all, but here are a few places to start if you want to learn more:
Bo and Bön: Ancient Shamanic Traditions of Siberia and Tibet by Dmitry Ermakov - In-depth descriptions of rituals, both to gods and for community, in Tibetan religions
Bön: Its Encounter with Buddhism in Tibet by B.L. Bansal, 1994 - If you're interested in a historical overview of the Bön/Buddhism meeting, clash, and cultural sharing, this is the book for you!
The Bön Religion of Tibet: The Iconography of a Living Tradition by Per Kvaerne - A good source for information on Bön and its deities as well as a detailed look at the art and iconography of the religion
Bön: The Magic Word: The Indigenous Religion of Tibet by Samten G. Karrmay - Similar to the Kvaerne one above, right down to heavy use of artwork to illustrate/add context to Bön beliefs and practices
Bön: Tibet's Ancient Religion by Christoph Baumer - A full look at the "classical" Bön religion, with lots of attention paid to how it's practiced today and a thorough historical accounting of its changes since the advent of Buddhism
Death and Beyond in Ancient Tibet by John V. Bellezza - A thorough look at funerary practices in Bön, including material from funeral texts
The Diamond Path: Tibetan & Mongolian Myth by Clifford Bishop, James Chambers & Michael Kerrigan, 1998 - A surface-level overview, but includes some nice art and comparisons between native Tibetan and Mongolian religions
Spirit-Mediums, Sacred Mountains and Related Bön Textual Traditions in Upper Tibet: Calling Down the Gods by John V. Bellezza, 2005 - Probably the dryest and most academic book on this list, but FULL of information about Bön practices
Tales of the Golden Corpse: Tibetan Folk Tales by Sandra Benson - These are folk tales, so the gods don't always make full appearances, and there's a hefty mix of Buddhism and more traditional Tibetan folklore, but it's a great way to get the flavor of Tibetan mythology and folk traditions
(Links are to WorldCat for those looking for a library copy, but of course you can also go scare one up for purchase if you want to!)
If you're just looking for a place to start, I'd recommend The Diamond Path as a starter and then Bön: Tibet's Ancient Religion and Spirit-Mediums, Sacred Mountains once you've gotten into it, but whatever calls to you as most interesting is probably the way to go. And I can't get out of here without suggesting EVERYBODY read The Epic of Gesar of Ling, the major Tibetan heroic epic and a story shared with a lot of other nearby cultures as well. It's very long - more than 20 times as long as Iliad! - but worth it. The best English translation currently is probably Alai's The Song of King Gesar, but there's an old translation version here (this is the Mongolian Buryat version, though, so you'll see different details and terminology than you would in a Tibetan version!) or check out Li Lianron's fabulous History and the Tibetan Epic Gesar for discussion of how it fits into the northern Asian mythology scene.
We can definitely say that we don't know as much about Bön as we'd like to, either, so we'll all be studying right alongside you!