Recommended books on theurgy and hermetics:
Initiation Into Hermetics of course, and I'd recommend going ahead and reading the theory sections of Practice of Magical Evocation and Key to the True Qabalah, they're much more extensive than the theory sections in IIH, and while parts might not make sense, they will still teach you a lot about the nature of the spiritual universe and spiritual entities, and the practice of constructing magical formulas, which is what real Qabalah is (the Tree of Life is just the background cosmology, unfortunately the Rosicrucians didn't have access to proper Qabalistic training, just a few introductory texts, so they mistook the Tree of Life for the whole of Qabalah, when it's really just the Jewish mystical cosmology).
The real Qabalah actually originated in Egypt, in the ancient world Egyptian magicians were famous for being able to use magical formulas to create fantastic effects, which is why Egypt gained a reputation as the land of magic, and why their magic was known as 'heka,' literally translating as 'naming' or 'speech.' The Jewish people picked it up (through Moses, apocryphally), and passed it down in a secret lineage among a small portion of their priesthood. Anyway, I'm rambling.
Also the IIH Companion by Rawn Clark, and the Commentary by Frater Veos. Both will make IIH significantly easier to understand and to work your way through. Clark's is more theoretical and Veos' is more practical, both are worth reading in full.
I don't feel comfortable recommending any other practical texts alongside IIH, since it contains everything you need and a lot of other texts will just confuse you, or you'll end up performing practices from too many different systems, or that aren't energetically compatible (I made this mistake, and it caused more harm than good).
For theory I'd recommend:
The works of Algis Uzdavinys, a recently deceased scholar who studied theurgy and philosophy. Especially:
The Golden Chain: An Anthology of Platonic and Pythagorean Philosophy
Philosophy as a Rite of Rebirth: From Ancient Egypt to Neoplatonism
The Heart of Plotinus: The Essential Enneads
Philosophy and Theurgy in Late Antiquity
Orpheus and the Roots of Platonism (The Matheson Trust, 2011)
These will be be real eye openers if you've ever taken a Philosophy 101 course in school or university, as modern "philosophers" tend to ignore the fact that the ancient Greeks who founded what they call "Western Philosophy" were strong mystics, and many of them practicing magicians. They find this very uncomfortable, since modern academia doesn't accept anything other than materialist reductionism, so they leave it out of their accounts of the Greeks and their philosophy entirely, outrageously misrepresenting them.
Theurgy, or the Hermetic Practice, a Treatise on Spiritual Alchemy - E.J. Langford Garstin. An interesting book, but be careful taking him too seriously compared to other sources, for reasons you'll find out if you read about the life of the author. However it still contains a lot of solid information and is worth at least giving a chance.
Three Books of Occult Philosophy - Agrippa. A bit dense in the reading, but good source for the background knowledge and cosmology you'll need.
The Corpus Hermeticum
The Emerald Tablet of Hermes - NOT "The Emerald Tablet of Thoth (sometimes "Thoth The Atlantean)." The Emerald Tablet of Hermes is a short set of hermetic and alchemical principles dating back to, as far as we can trace it, the 8th Century, but almost certainly based on or taken from older texts. The "Tablet of Thoth," by contrast, is a new age fakery supposedly received by channeling, and is full of nonsense.
The Hymns of Orpheus
The Enneads
Nicomachean Ethics
Plato's Republic
The Illiad and the Odyssey (both of which contain strong theurgic parables hidden inside them)
I won't make any specific recommendations here, since there's so much out there, but reading as much as you can on Greek and Egyptian mythology until you have a good grasp on both is a good idea, as it will give you the mythos within which they hid a lot of parables and teachings.