As a non fiction writer (mind, body and spirit/religion) I have a large collection of books in both of these genres. I love the Conversations with God series and the works of Eckhart Tolle in particular, but have books on all different aspects of spirituality - books on meditation, on crystals (I am a qualified crystal therapist), Reiki (also do this), spirit guides and all sorts of things.
I also have a large collection of religious books - the complete works of Josephus, a Greek Hebrew Study Bible, the Nag Hammadi and other Gnostic texts etc.
Then there is the 'aternative' history books - Andrew Collins, Henry Lincoln, Lynn Picknett and Clive Prince, Lawrence Gardner, David Rohl and so on. My favourite author in this genre is Graham Phillips - who has written some fabulous stuff over the years - The Moses Legacy, Act of God and so on. I have met Graham several times and he is a really funny man and fascinating to talk to, since he is so knowledgeable on so many things.
I also have a large collection of Icelandic Sagas - translated from the original Icelandic/Old Norse of course. These are tales written in the 12th and 13th Centuries about the settlement period of the country, which took place towards the end of the 10th Century. Fascinating stuff with tangled relationships and blood feuds. I will be adding to the collection next year I expect, since I hope to go back for a holiday - I have been five times and my last visit was in 2001.
I do have some fiction books as well though - which I have recently begun to read again, after writing and reading non fiction for so many years, and I find that I really enjoy having something lighter to read. Mind you, a lot of the stuff I like could not be considered all that light, since I prefer the books about life in different countries, with a dark element, women in Islamic countries for example and what it is like for them.