I was correcting homework today for a very serious student taking multiple courses when I realized that Lilly hadn't really covered a particular question, but I had the solution, Bonatti!
I knew that this student would benefit from getting exposed to Bonatti and was both willing and had the resources to add Bonatti to their library.
Then as I thought about it, I realized that Lilly, Bonatti, Ramesey and Al-Biruni were my key texts for traditional astrology. That's William Lilly's Christian Astrology, Guido Bonatti's Book of Astronomy/Liber Astronomiae, William Ramesey's Astrology Restored/Astrologia Restaurata and Al-Biruni's Book of the Art of Instruction in the Elements of Astrology. While I do have a much larger library of traditional astrology texts, these are the ones I rely on and actually refer to in my practice. Lilly for horary and natal, Bonatti for horary, natal and electional, Ramesey for electional, Al-Biruni, as a basic astrological source.
For astrological magic, the key texts are Picatrix and Agrippa's Three Books of Occult Philosophy.
If you are serious about traditional astrology, you need to have a copy of all these books!
Lilly, Horary books 1&2
Lilly, Natal, book 3
Ramesey, just google search the pdf, or if you like having a printed copy, like I do for reference, plus a bunch of other electional books you can get my Renaissance Astrology Electional Compilation (this is the textbook for my electional course)
Bonatti, you have to get these individually, but well worth it
These are also available directly from the translator Ben Dykes, thanks a million Ben for making these available
or Eric Perdue's excellent translation of Book one
IMHO you definitely need these texts, but once you have them, you've got about 90% of what you really need.