Having bought Lonely Planet, and Frommers, some time ago,and been given (by Santa) DK, Rough Guides, AND Insights, (guess the family know we are off to Japan soon!!!) I can thoroughly recommend Lonely Planet, and Insights, and fully agree, Armand V, about the JNTO website guides: they are amazing. LP is the most detailed by far, and the maps are great: would suggest getting the individual small LP books just on Tokyo, and Kyoto, if that's all you are going to see: they have all the same info as the full Japan guide, but are much easier to carry around, and have the better maps of those cities.
Good luck!!