Because I like playing with spreadsheets . . .
I finished reading 55 books in 2015; 53 were new to me and two were rereads. (I'm ignoring the rereads for the rest of this post, mostly because they're on a separate tab of my spreadsheet.)
The oldest was Thoreau's Walden, published in 1854. I read five books from the second half of the nineteenth century (Thoreau, Gaskell, Eliot, Wilde, Stoker) and four from 1962-65 that make an interesting cultural microcosm (in publication order: Dick's The Man in the High Castle, Plath's The Bell Jar, Rendell's From Doon with Death, Le Carre's The Looking Glass War). Then one from the 1990s and the rest from the 2000s, including 15 published in 2014 and twelve published in 2015. (There are also three books published in 2015 that I read in 2014, which keeps confusing my various lists.)
The longest was George R.R. Martin's A Feast for Crows at 1060 pages; the shortest was Deanna Raybourn's Midsummer Night at 56 pages. The books add up to a total of 19263 pages, or an average of 363 pages each.
(According to what I could find from author bios) I read 37 books by American authors, ten by British, three by Irish, and one each by Scottish, Swedish, and Japanese authors.
As far as I could determine: 36 by authors identifying as female, 15 by male, one by a male/female writing pair, and one anthology including authors of more than one gender.
I didn't record how many authors of color I read, but I'll try to be more aware of that next year.
33 books published/marketed to an adult audience, 19 young adult, one juvenile.
45 fiction and 8 nonfiction. (I'm being generous to Thoreau there.) The fiction numbers include ten mystery, six fantasy, five science fiction, three historical fiction, three romance, and two horror.
20 of the books were part of series. Nine were first in the series, six were second, three were fourth, one was 3.5, and one was 12.5. (The fractions are interstitial novellas. I don't make the rules.)
The only authors by whom I read more than one book this calendar year were Tana French and Deanna Raybourn. (Both highly recommended, obviously.)
36 authors were new to me (plus maybe a few more in a short story anthology; I don't have that list in front of me).
33 of the books were from the library, nine I bought specifically to read right away(ish), seven I already had unread on my shelves, three were gifts, one I borrowed from a friend, and one was a review copy from the publisher.
31 hardcover, 15 trade paperback, three ebook, two mass market paperback. One paper ARC (advance reading copy) and one electronic ARC.
Next up: A list of what all these books actually were, if you want?