First, and most important, make sure your ink is waterproof. Not water-resistant. Waterproof.
Yeah, it seems obvious, but a lot of folks forget to check and end up ruining a project.
I know others have suggested coffee, but I'd like to put a word in for tea. In particular, Nestea instant tea, Decaf, Unsweetened. The problem with coffee is that it has a noticable odor (that might not dissipate inside a closed book) as well as natural oils that can react with printer inks.
Mix up the tea at double strength with a drop or two of dishwashing detergent, shake vigorously, then filter it through a coffee filter into a plant mister or perfume atomizer. Spritz the pages, sprinkle a few crystals of the dry instant tea powder on the page if you want some mold spots, and let dry. As mentioned previously you may want to use wax paper or cling wrap to protect the pages below the ones you're working on. Once you've aged all the pages you can get a nice edge oxidation effect by closing the book and painting the edges of the paper block with the same tea solution. Then let the book air dry. Use the steam from a pot of boiling water or a tea kettle to help loosten any pages that may be sticking together after the treatment.
If you want a *really* weathered edge treatment you can then fume the book with ammonia for a week or two.
Cordially,
MM
Note: If you want to try aging the whole book at once you can use the ammonia fuming technique, but that will only work on cheap books with wood fiber paper. Just set the book and a cup filled with a strong, ammonia based cleaning solution inside a sealable storage container (Tupperware or Rubbermaid type stuff) and let it soak up the ammonia for 7-14 days. That's normally long enough for the natural acids in the paper to react with the ammonia and kick off the yellowing process.