Bento Boxes: Japanese Meals On the Go is, I believe, the only book available in English at the moment that is wholly dedicated to Japanese bento box lunches. I didn't have much incentive to get this for myself, but someone kindly sent me a copy to take a look at recently, so I can finally review it properly.
This slim softcover book (64 pages) is published by a Japanese publishing company. It's quite obviously a translation from a Japanese book, one I am guessing published about 10 years ago (the publication date of this English version is 2001). I'm not familiar with the author, Naomi Kijima, though that doesn't mean much. The bentos are very attractive, if a bit old-fashioned in feeling, and the photographs are beautiful.