Lately, it's been anything which I find challenging and make me think and want to investigate further, e.g. philosophy, physics, cosmology, authors who attempt to prove the existence of some kind of god.
I am a buff for reference materials and factual representations. Art, culture, economic, science, nature, forensic, history, architecture, geology, etc., that in my mind is worthy as an ageless reference resource. I find more enjoyment having the hard copy. Am working on a reference book for Japanese art swords and related items as i am into that at this time.
Especially the ones that concern the political or religious turmoils of either our world or an alternative one (e.g. The Lord of the Rings, A Song of Ice and Fire, The Count of Monte Cristo, His Dark Materials).
However, I also enjoy books about physics, especially quantum mechanics and other new, unexplored fields of science which the science community has just begun investigating.
In simple words, any book which gets me thinking and questioning what I already know.
I am reading as much as I can find (and attempt to understand) about physics, both in quantum mechanics and cosmology. I have read Brian Greene's books which he tries to explain in "layman's" language, but even those are quite challenging. He has narrated some shows on TV in a "Fabric of the Cosmos" series which are quite well produced with recorded input from several contemporary physicists. Have you seen those?
They are similar in content, but have different producers. Through the Wormhole appears on the Science Channel on my cable outlet on Wednesday nights. The "Fabric of the Cosmos" was a series of shows which was produced by NOVA and shown on the PBS stations on my cable channels. It was based upon a book which Brian Greene wrote in 2005 by the same title which described the latest knowledge of physics at that time in terms that non-physicists could understand. I did try to read the book and it was quite challenging, but the TV presentation was much easier to comprehend with all the special effects visual format. The "Through the Wormhole" series gets into a broader range of topics which goes into sciences related to psychology and medicine and some philosophy as well. It is well done and I have been watching all of those.
Books about world religions and associated subjects and books related to jewelry, gemstones and similar topics. Sometimes horror or mystery grab my attention.
That's a toughie for me. I have quite a few I like alot...horror, historical fiction, and bios are among my favorites. I think it depends on the mood I'm in at the time..what I feel like getting into.
However, I also enjoy books about physics, especially quantum mechanics and other new, unexplored fields of science which the science community has just begun investigating.
In simple words, any book which gets me thinking and questioning what I already know.