For as fast of a reader as I am (and I can read really really fast), I have noticed I don't read all types of books at the same speed, or with the same enthusiasm. Here are a couple of categories I have created in a completely arbitrary manner into which my reading speeds/book types fall into.
Dense - I love me a dense book, I like them thick, rich, detailed and thought provoking. Books like this tend to take me a little longer to read, usually because I have to stop and process the idea or think through the plot and make sure I understand where everything fits together. These are books that you read a paragraph or a concept and just have to put it down and chew it over for a while. These books can take me up to a month to read.
What is the Big Secret?!? - These are the books that may or not be that good, but hint at a big twist or reveal. These books tend to get read very quickly, usually so I can figure out just what the big deal is. A lot of times these books actually frustrate me because most of the book is just about setting up the twist or reveal and I am just trying to get through it to find out what the big secret is. Plus side is I can usually power through these in a week or less.
Have to Finish It - Sometimes I start a book and I just have a hard time getting into it. There is nothing objectionable to it, I don't hate it, I just am not that into it. For some reason though, I feel guilty about starting books and not finishing them. These books tend to take me FOREVER to read, sometimes I even put them away and try again later. That being said, sometimes when I pick it back up again, or finally get into it they turn out really really good.
Page Turners - These are my favorite books, these are the books I can't put down for any number of reasons. Sometimes the writing is just so phenomenal I start reading, look up and find that several hours have passed. Sometimes I start reading, get forced to put it down and then can't stop thinking about the world, story or characters. These are the books you look forward to picking back up as soon as you can, the ones that make you angry when you get interrupted and the ones that are always over way to soon. I usually finish this type of book in 1-3 days.