The book is not actually a sequel to three books of occult philosophy. Most of it is not written by Agrippa. On Magical Ceremonies may have been written by him and is largely a practical summation of the information provided in Three Books of Occult Philosophy. Three Books of Occult Philosophy essentially presents the system of magical thought from the grimoire period, but it is a pretty large book and takes a lot to get through. If you want to get started, On Magical Ceremonies is a lot more accessible.
These texts collect together manuscripts by the somewhat mysterious Dr. Rudd. Rudd most likely was around a generation or so after Dr. John Dee. Like Agrippa and the Magical Calendar Rudd's work explains many correspondences useful in medieval/renaissance spirit magic. It also has Rudd explaining various concepts of magic such as talismans and image magic, explaining various types of spirits, various symbols of astrological magic and other similar topics. Rudd also presents info on spirits less commonly addressed in modern approaches to spirit magic, and he is a good example of linking Scholastic Image Magic with Grimoiric Ritual Magic. Rudd's work also includes Rudd's own grimoire and his approach to blending Enochian material into grimoire magic.
4. Art of Drawing Spirits into Crystals by Johannes Trithemius
This is a pretty short text which you can get online. This presents a very simply form of crystallomancy or conjuring a spirit into a crystal. It's based on working with the seven planetary spirits but the system can be used for any sort of spirits. It uses very few tools, and not a lot of rituals, just a few simple prayers. In particular it is a very good introduction to how an altar has specific elements for manifesting spirits in grimoire magic. For a lot of magicians this work inspires their standard means of working with spirits.
5. The Clavis or Key to the Magic of Solomon by Sibley and Hockley edited by Peterson
This book is pretty big and expensive. But it is fricken beautiful. It has images of the original manuscript pages, and these take up the bulk of the book. The manuscript pages have beautiful illustrations of the talismans and the book is printed on a really nice glossy paper. On the negative end, the actual “critical edition” of the text is only a very small portion of the otherwise huge book. That said, the book is essentially a pair of grimoires, with a handful of spirit conjure rituals sandwiched between them. The section by Sibley is very useful because unlike most grimoires which just collect prayers and descriptions of tools, Sibley is writing something halfway between a grimoire and a book about grimoire magic. Sibley was writing only a few generations before the occult revival that produced the Golden Dawn, so by this point books about magic, as opposed to simply books of magic, were starting to appear so Sibley gives a much more accessible series of instructions than we find in a lot of grimoires.
6. The Key of Solomon the King edited by S.L. MacGregor Mathers
This isn't the best version of the Key of Solomon available but it was the standard version everyone had for a long time. This was what I used for my first formal approach to spirit conjuration. It provides talismans for a lot of different purposes, and clear instructions for making tools and putting together your ritual. This is kind of the baseline of Solomonic instructions.
7. The Veritable Key of Solomon edited by Skinner and Rankine
This is probably the best version of the Key of Solomon currently available. This collects several different variants of the Key of Solomon. In addition to providing a full exposition of how to create the tools and how to do Solomonic Magic, but more importantly it provides several ritual instructions aside from the main conjuration method, and provides several different purposes for the system far beyond what one might generally connect with Solomonic magic at first consideration. It shows the breadth of Solomonic magic as a system.
So, in addition to these, again, I would recommend Agrippa's Three Books of Occult Philosophy. But don't let that slow you down in getting started. Read a handful of these, especially the ones that are easily accessible, and then start doing the work. As you do the work, keep studying and reading and expanding on how you work. Look into other traditional forms of magic like the Papyri Graeco-Magicae. Traditional Witchcraft, and traditional forms of European and Caribbean syncretic sorcery because these can also present ideas that are useful for consideration in spirit conjure and sorcery.
