For me, I am going to say Initiation into Hermetics (IIH). I am a big Bardon fan, albeit not frothing at the mouth for him. I appreciate having the level of explanation and analysis on many different parts of magick, even if it is done from a western perspective. There is a rich level of explanation going on behind many aspects of magick and spirituality that he gives us and that makes a lot of sense to me. Additionally, it is also the level of practical stuff that he puts into our hands and allows us to play with in a structured way. Stuff like meditation, visualization practice, and energy work all in one place is pretty nice to have, especially when it is in a (mostly) basic plain language that people can understand. It takes a bit of getting used to, but this stuff is quite lucid to read compared to some of the Crowley stuff. It is also less dogmatic towards a particular belief system, so it can be adapted (to an extent) to a persons own belief system.
As a training manual for more complex occult topics, it is complete and sufficient. It isn't perfect and definitely not easy, but it is a worthwhile resource for those who are going to practice magick. By looking into the Bardon community, you can get a pretty good sense of where this book's inspirations come from and what is emphasised. There are a lot of influences from stuff like Buddhism, Hinduism (especially Yoga), Rosicrucianism, and more. It is a wide variety of influences that you are tapping into through a book like this, even if you would not be able to pick out all of them.
Other than IIH, anything that provides a less well known form of magick is valuable. We know a lot about western grimoires, ceremonial magick, witchcraft + wicca, and a lot about Eastern spiritual systems. But resources from less focused on traditions such as ATRs, Arabic magick, etc, can be extremely valuable in their own ways. If you pair these unusual resources with a background from IIH, you would be in a powerful position to innovate new and useful magick from these sources.
If you experiment from this innovation and create your own rituals within a grimoire / book of shadows, then I guess that ends up becoming the most valuable book you own.
