A fascinating topic of great personal interest! Another poster mentioned "Tantric Thelema." In addition to that book there was much information included in Kenneth Grant's Typhonian tradition, (if one can sift the gold from the dross).
You probably know that the forum Lashtal, dedicated to Aleister Crowley, has some interesting discussions pertaining to godforms in a Thelemic context. Here's an initial search there:
https://www.lashtal.com/forums/?wpfs=godforms
Moving to non-Thelemic sources, Jason Miller's work provides practical spellcasting information rooted in both Tibetan and Western Magical traditions, so may offer some guidance:
https://www.strategicsorcery.net/
Material from the Cultus Sabbati tradition may be hard to source (and costly), but is highly relevant:
The Ku-Trishula or ‘Order of the Negative Trident’ is an inner cell of the Cultus Sabbati serving as an operant sanctuary for various tantric empowerments originating in Hinduism, Vajrayana Buddhism and Bön, adapted for the diverse contexts of the western magical tradition.
http://xoanon.co.uk/sodalities/
It would be impossible to fully address this topic without diving deeply into Graeco-Egyptian Theurgy, especially Iamblichus. There are many good books on theurgy available on Amazon, including the works of Gregory Shaw and Algis Uzdavinys. Another theurgic favorite is "The Mithras Liturgy," expanding on a chapter from the PGM.
The Egyptian theurgic tradition, a primary source of western magick, also includes the various editions of Books of the Dead, Pyramid Texts, Coffin Texts etc., which most definitely include "assumption of godforms" as a vehicle for traversing the Duat. Stick to the academic texts rather than the popular books written for laypeople.
Godforms are often indistiguishable from archetypes. Carl Jung's Red Book is justly famed as a modern gnostic grimoire exploring the deepest archtypes and which offers psychological insight into the same theurgical matrix. The following work is far less expensive than the full Red Book:
Finally, it will not surprise you that no tradition existing today possesses more information about assumption of godforms than Tibetan Buddhism.
Possibly the single piece of Buddhist information most relevant to western magical practitioners is the traditional reason for assuming god forms in the first place (as opposed to simply visualizing one's own enhanced astral body). Yogis assume the appearance, perceptions, powers and 'divine pride' of a deity, "In order to prevent ordinary appearances from arising."
"Ordinary appearances" should be taken to refer to our normal ego-neuroses, rooted in our own self-image, which can be magnified to life-damaging levels through performing magical/yogic practices. Hence assuming non-Self-reflecting forms, and why the rule is to never practice without the guidance of one's teacher. Many yogis and magicians have taken bad falls on this path. Caveat Emptor!
Back to sources... even though the most detailed information on secret mantra is reserved for students who have received teachings and empowerments from their Lama, there are many source texts available on Amazon. If these particular teachings are new to you, start with "Clear Light of Bliss," an advanced practice text explaining the "how" and "why" of a godform-related practice by maverick spiritual teacher Geshe Kelsang Gyatso.
Though every individual school of Vajrayana works with different Yidams, their basic techniques are generally similar to each other, and all describe the imaginal or rainbowlike nature of the godforms as metaphors for our own Buddha nature, which is "empty" of inherent existence.
There are some interesting parallels between the Vajrayogini godform and the goddess Babalon in Thelema. Machig Labdron's profound Chod practice (with the Vajrayogini emanation of Machig as a powerful feminine godform) has a lot to offer in this area. Western-born lama and lineage-holder Tsultrim Allione has written the most reader-friendly discourses on Chod from a traditional point of view, especially in "Feeding Your Demons." For enlightening info on Babalon Herself, Daniel Schulke's anthology, "A Rose Veiled in Black," Manon Hedenborg White's "The Eloquent Blood: The Goddess Babalon and the Construction of Femininities in Western Esotericism," and "The Red Goddess," by Peter Grey are all highly recommended.
https://threehandspress.com/shop/a-rose-veiled-in-black/
https://scarletimprint.com/publications/the-red-goddess
Lastly this online forum for serious Dharma practitioners is an excellent resource:
https://dharmawheel.net/viewforum.php?f=40
Some final thoughts: Besides the siddhis as a side-effect, successful practice of deity yoga (or godform magic) is said to lead to realization of the essential Ground of Being as indistinguishable from the empty nature of the self. Such an identity-dissolving awakening seems echoed in the ego-killing experience of Crossing the Abyss in Thelema.
Out of respect, we won't name the fierce Buddhist guardian deities here, but there seem to be striking (though unintentional) parallels with the ego-destroying, anticosmic demon Choronzon, guardian of the Abyss. And just as with those Dharma Protectors, no one dares to choose Choronzon as a personal godform either (with one famous exception that had unpleasant results).
Edit: Already a wall of text far longer than planned, so left out references to the Qabalistic, Muslim and medieval European schools of Solomonic angel magic. These names and forces may- or may not- be evoked with self-identification so strictly speaking are not always "assumption of godforms." But as fruits of the same primordial tree(s) they are worth exploring in your search as secondary sources or objects of comparison.
Hope that some of this helps!