Reddit mentions of Spirit Walking: A Course in Shamanic Power

A good place to start I think is reading Shamanic Voices by anthropoligist Joan Halifax. It isn't a how-to guide or anything, but gives intimate accounts of Shamanic practices throughout the world. It includes records of rituals performed by Maria Sabina that you may find especially interesting.

After that, I'd recommend Shamanism: Archaic Techniques of Ecstasy by reliigious history professor Mircea Eliade. Its a bit thick but is very thorough in its treatment of Shamanic practices through the millennia and around the world, including descriptions of numerous techniques used for entering trance states, cosmologies, symbolism, initiations, and powers claimed by Shamans. This is an academic work, however and won't give you step by step instructions (if that is what you are looking for).

If you are looking for something a bit lighter, Supernatural by Graham Hancock is an interesting read. In it he looks at parallels between drug-induced experiences, Shamanism, fairies, and reports of extraterrestrials. If I had known that last part before I read it I probably would have skipped this book but he actually made some very interesting points that I think makes the book worth reading. Also, he relies heavily on Joan Halifax's book as a source and spends a decent amount of time discussing Maria Sabina and psilocybin usage.

The beginner how-to department is an area I'm less versed in but I've heard good things about this book and its companion. Personally I'd generally recommend getting oneself intimately familiar with current and past Shamanic practices through the academic works on the subject and then creating a personalized system - though commercial how-to guides can certainly provide some practical hints and inspiration.