Bearing in mind that I'm older than dirt, and a couple of these are mostly older books--
1. The I Ching
2. The Touch of Healing by Alice Burmeister
3. Christian Astrology by William Lilly
All of these have opened my eyes to a whole universe of new possibilities. All three came into my life in unusual ways: two of them fell off a bookshelf right in front of me (one of them gave my big toe a big bruise), and the third kept appearing wherever I went, even though it had been out of print for years. Finally, I got the hint and bought it.
Here's why for each of them:
1. I-Ching (toe-bruiser)--At the time, I wasn't inclined to delve into such a large book, and when I made my first attempt, it seemed like "Greek" to me. But as I continued reading, it sucked me in, and not only was I hooked, but I started seeing life in a totally different way. I do not use it for divination (although some do, and successfully at that), but as a little morning lesson to ponder every day.
2. Touch of Healing (foot bumper)--The ONE book for the "energy" medicine cabinet. Amazing to discover that I didn't necessarily need meds for most things. Opened up a whole world of feeling better without side effects, especially when I use it with my Reiki training.
3. Christian Astrology (the stalking book)--When it appeared in my life, I had been taught that you should never use astrology for prediction, but only for character analysis. William Lilly, in his quaint language, taught me otherwise, and deepened and widened my understanding of how the universe works, and how to work with it as opposed to against it. The I Ching and this book have much in agreement, although they address different aspects of what we now call "the field."
I've read a lot of good books since, and in between, but I'll go with these three for most life-changing.