Although currently out-of-print, this is the book to have in your pack while hiking or biking. It's the most detailed towpath guide available. It lists everything there is to see, as well as many close by significant landmarks with lots of historical background. It's also filled with maps and photographs. Although the photos are all black and white, most of them are historical and depict the prime of places that have since fallen into disuse.
Thomas Hahn has written several books on the canal but this is the most widely available. It's on sale at every NPS store and at Washington DC bookstores and outfitters. All the mileage markers on this web site is from this book.
If you can wait for it, an updated version of The Towpath Guide is being worked on.