The Key of Solomon the King (aka “Clavicula Salomonis”). There are a bunch of translations out there, but this is one of the basic texts on summoning and controlling demons and angels.
The Lesser Key of Solomon (aka “The Goetia”). Along the same lines as the Key of Solomon above, but this one is actually a different book. This is the one that has the 72 Goetic demons, along with their various seals. Alistair Crowley did his own edited translation of this book, which was quite different from the original text (some say it’s much more practical in terms of application).
The Black Pullet. Another early-modern grimoire, covering much the same material of the two Keys of Solomon, but with it’s own details and symbols. Personally, I think this is one of the best names for a book of magic ever.
Futhark, A Handbook of Rune Magic by Edred Thorsson. This was one of the first books to popularize the runes within the pagan and occult communities, and remains one of the best in terms of the quality of the scholarship (the author holds a PhD in Germanic Languages and Medieval Studies) and accessibility. It covers all of the historically-known runic alphabets (there are more than just one) and describes magical uses for the runes including divination, magic, etc. If you get one book on runes, make it this one.
The Black Books of Elverum. This is a nicely different pair of magical books; two grimoires from Norway. The book gives hundreds of charms and spells to find stolen objects, get a lover, etc. etc.
The Magician’s Companion by Bill Whitcomb. A pretty comprehensive encyclopedia of everything to do with ritual magic, broken up into a number of different magical models; 4 elements, 7 planets, 28 lunar mansions, etc. An excellent source-book that you can just open up at random and find inspiration.
The Anglo-Saxon Metrical Charms. Want to know what folk-charms and spells look like? Go to the source. Here you’ll find what the ancient Anglo-Saxons did to cure various diseases, find lost cattle, etc.