I recently read a book that was printed with brown ink. I have noticed a number of current books are beginning to be printed with the brown ink. It does remind me a bit of old sepia toned photographs, and I think the idea is to create an aged look to the book. This can help set the tone of the novel, subtly transporting the reader to a different world or time.
After reading the book and considering the brown ink, I began to realize how much a book's aesthetic really does make a difference. Certain fonts are more difficult to read, and tend to scream "self-published" to me. Also, the type of paper and the largeness of the print can contribute to the pleasantness of reading a particular book. Even the largeness of a particular book can limit the readability. (Trying to read War and Peace lying down, for instance, isn't something I can recommend.)
Books aren't just about the words on the page. There is something that contributes or detracts from the experience that is part of the physical nature of the book.