Initiation into Hermetics. I admit I am a long time Bardon fan, but I have always found IIH to be deeper than what it directly says. It is about as close to Chaos Magick fundamental as you can get for western mystery tradition without necessarily sacrificing the specific symbolism of western magick. I found a lot of the practices that it describes to be quite applicable to a number of different magical systems and spiritual beliefs. if you really think about what the book is saying and cross-research other systems, you will find many parallels. The descriptions in IIH actually consistent enrich and enhance my understanding of other practices in different systems.
To give an example, a practice such as wudu / ablution, found in Islam, is giving new meaning when you look at it from the lens framed from Bardon's discussion for the magick of water. Ablution is described as a ritualistic purifying wash of the body prior to prayer within Islam, but the esoteric interpretation of this is left unsaid beyond the basics. The religion says that it is both a symbolic and a physical cleansing of the body prior to holy activity and this is true, but what does that actually mean? In Franz Bardon's view, water has a an attractive capacity to it that is capable of pulling different energetic influences into it, especially through the conscious application of intent through will. Cold water in particular is encouraged as a form of purification by being especially effective in pulling out the "negative" astral traits from our subtle bodies. When you combine Bardon's explanation of this with the surface explanation of ablution, you now see the purifying purpose of ablution as being the use of the water element's attractive properties to cleanse yourself of astral negative energies prior to prayer. It is a subtle addition, but a powerful piece of understanding that shows you the power of magick at work even in contexts where people do not supposedly practice magick.
This just one example of course, but I do find the explanation of Bardon to really give effective keys in understanding many different spiritual practices in other systems. You might also be able to get these keys through Chaos magick books, but to have them conveniently wrapped up in the western symbolism that many of us are familiar with is quite valuable. IIH's practices also help to enhance this even further and are quite transferable to several other ritual systems if you superimpose the system's symbols onto Bardon's practice.
Of course, I do admit that IIH has its flaws, but it is a much deeper work than what most people give credit for. I do not believe it is a divine system of instruction unlike what the book claims, but I do believe it is among the most coherent systems of instruction. It has informed my own practice of magick in many different ways and I owe it a lot. It has its criticisms and many are valid, but many other critiques show an obvious lack of understanding about the system's intricacies and intentions.