The following are books written In English which helped me to understand manga better, dictionaries and the like. You can purchase any of them through Amazon. Be sure to check the other resources section as well, for example the Fictin and Miscellaneous resources.

The Kanji Dictionary

Probably the best dictionary of it's kind. If you can only afford one dictionary, this should be it. The best part about this dictionary is that you can look up any words by simply identifying one kanji which makes up the compound.

A Dictionary of Basic Japanese Grammar

I had to use this and the intermediate grammar book at university. It's very concise and I liked the way the organise the grammar structure, as well as the sentence patterns that they use.

A Dictionary of Intermediate Japanese Grammar

Almost the same as the one above, except it contains more grammar structure. While covering some of the ones already covered in the first book with more details.

Dreamland Japan : Writings on Modern Manga
by Frederik L. Schodt
A truly comprehensive book about several genre of manga. It contains detailed descriptions on some manga magazines as well as current mangaka. Buy it! or if that's not possible "borrow" it from a friend :).

Manga! Manga! : The World of Japanese Comics
by Frederik L. Schodt
A look into how manga had evolved from the 12th century to now. Contains a brief look into a wide range of "classic" manga like Rose of Versailles.

Japanese for Everyone : A Functional Approach to Daily Communications
by Nagara Susumu
Useful for beginners and it becomes even more so as you improved. It's very user friendly with dialogue and sentence structure discussed in detail. As your skills improved the book introduced more kanji to familiarise the readers.

Beyond Polite Japanese : A Dictionary of Japanese Slang and Colloquialisms
by Akihiko Yonekawa
If you're sick and tired of seeing recurring words that does not match its literal meaning you might want to have a look at this book. It contains useful slang common to manga, it makes reading just that much easier.

Random House Japanese - English English Japanese Dictionary
by Seigo Nakao
A dictionary :P. Handy to use and contains most of the vocab you will need.

Martin's Concise Japanese Dictionary : English-Japanese Japanese-English : Fully Romanized With Complete Kanji & Kana
by Samuel E. Martin
Pocket sized dictionary invaluable if you're going to do homestay as it's small enough to slip into your jacket pocket. It's easy to use and though contain less words than the Martin's dictionary it's still handy.