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.
However there are a few problems with this book. As I said, it’s a translation of a Japanese book and seems to be aimed at a Japanese audience of fairly experienced home cooks. Many of the bentos feature ingredients that are not that easy to get outside of Japan. This would put off most people from trying out the recipes I think, unless they were really determined. A few of the recipes even intimidated me a bit - even though I do have big stock of Japanese staples, some of the fresh ingredients are hard if not impossible for me to get my hands on.
Of course you do need Japanese ingredients to make Japanese food, but one of the major aims of both this site and Just Hungry  is to try to gently incorporate these Japanese ingredients with more widely available ingredients, for people who aren’t Japanese or don’t live in Japan. With a directly-translated-from-Japanese cookbook, that kind of thing isn’t taken into consideration.
The other problem with the book is that the recipes are quite abbreviated, so that a Japanese cooking beginner would be left with many questions.
All in all I’m not sure I’d recommend this book wholeheartedly, unless you live in Japan or in an area with easy accessibility to Japanese ingredients. It’s a bit in-between…too advanced for a Japanese cooking beginner, yet lacking detailed instructions. On the other hand it’s not expensive, so if you want an inspirational book to get your bento-creating juices going, or a good visual guide to traditional style Japanese bento lunches, it might be worth adding to your collection. (The bento styling looks a tad old-fashioned and stuffy to me though. The Japanese bento books I recommended earlier  are much fresher and modern to my eyes. This is a matter of personal taste of course.)
I would definitely supplement it with a basic Japanese cooking book in any case. In that category there are more choices in English, such as the one I reviewed some time ago  (by one of my favorite Japanese cooking gurus).