There are lots of bento-related books published every year in Japan. While most of them have plenty of colorful pictures, some are too wordy to be really useful for people who don't read Japanese. Here is a list of books that I have in my collection that I think would be very useful even if you don't read the text. Most of these books reflect my preference for books about healthy, vegetable-centric bento, mainly aimed at adults.
I'll be updating this page from time to time, so please check back occasionally. You can also see other, less annotated book recommendations in the Amazon Japan aStore.