I vote other because I use them all. My "stack" sits right beside "my chair" along with a butterfly ID book. When it gets too dark to garden outside, I sit and "garden inside". Sometimes l look up specifics and sometimes I dream and plan. My books are supplemented by DG, google and my trusty computer. I guess if I have a favorite, it would be garden design but reading that, generally leads me right back tto the other books and to the net. I read a thread recently on DG by butterfly chaser about getting a Kindle Fire so I wonder next year if we asked the same question, will people say Kindle instead of books.
I went with field guide, although I have all the others as well. I tend to use the internet for specific information (care, bugs, whatever) for a particular plant, but rely on my native plant field guide to try to figure out what I've got growing out and about on its own.
Beginning gardeners should start with a book(s) written for their region which covers "the basics". Another resource is the land grant university of their state and the county agricultural/horticultural agent's office. Many have free or low cost publications in print or on-line and most have a Master Gardener group to help via educational presentations. Local garden clubs, local nurseries and neighbors that have great gardens are another resource.
My daughter's husband was in the Navy for a number of years, they lived on one coast or the other as duty stations changed. As soon as she was settled in, she would drive around the neighborhood looking for "gardeners" to help her with "how-to" information for the new location!
Although I said, "A book for the culture of a specific plant.", it is really a bit of everything. I have always relied heavily on my books and have been collecting for about 30 years. I have everything from Greenhouse & Nursery production, Propagation, Landscape Design to plant specific book like Cycads & Palms, Fuchsias, Heliconias, Camellias to regionally specific books like the Western Garden Book that Katlian mentioned, native plants of British Columbia, Trees of Vancouver/Seattle, etc. As I hang out in the ID forum alot, books with mostly images are helpful in flipping through the pages to get an ID if I remember having seen it somewhere in one of my books. I study the Dirr books, Hillier Manuals and the Encyclopedias for tidbits of info.
I am a bit surprised that 'field book' is so far winning the popular vote. I take that as a good sign that we are all becoming a bit more in tune to our own micro-climates and working along side nature rather than against it. All good. Nothing against out-of-zone exotics, mind you, but I for one am finding a more naturalistic garden to be more soothing to my eye and less labor intensive.
I agree with growin. My personal library is filled with all types of plant/garden books and guides but my faves tend to be the encyclopedia types that cover a vast assortment of plants. From there, I can hone in on specifics. Armed with the right information, it then comes down to trial and error as well as testimony from other gardeners with what works for them. I think it's all about the challenge!
I agree with whoever wrote "all the above". I have a huge assortment of books that fit in all the categories. However, if I just want to look up something real quick, I go for my Encyclopedia of Organic Gardening.
I didn't get to vote and now can't find my way back so will just say My favorite book is Arkansas Wildflowers as I live in SE Oklahoma but very close to Arkansas 'The plants covered in the Arkansas book matches up with my area much better than any of the Oklahoma field guides that i own ..FYI I bought it at a Arkansas state park (Queen Wilhelmina)