I thought today I'd show you my Japanese book collection. I could only find about half of them though because the other half are in storage for when I go back to University :( I found loads of random books in Japanese that I'd completely forgotten I had, so this was quite a good exercise for me XD I am not claiming this is the definitive 'best' of Japanese learning books, but these are what I find useful and hopefully it'll give you an idea in what you should invest in.
To start of we have the dictionaries and other learning materials. (I haven't got photos of everything)
As a whole, I really rate Kenkyusha's dictionaries. The only guideline I'd really recommend for purchasing a good dictionary is that THE JAPANESE IS NOT IN ROMAJI. The Oxford dictionary I think was the only one that I found in a bookstore that was in kana then kanji. Romaji dictionaries will not help in your fluency of reading or your grasp of hiragana. Forcing yourself to read kana from the get-go, no matter how hard you find it to begin with, is seriously (in my opinion) a vital foundation. My younger sister learnt Japanese from a book that was in romaji and by the end of GCSE still really struggled with kana, whereas I'd never read Japanese in romaji and could read and write it without any problems.
Unfortunately the grammar dictionary does have the sentences in romaji then kana/kanji which does irk me slightly, but its still very useful!
These are the textbooks I use for university.(Some of them, not all of them! BTW this is just year 1, crossover to year 2 just to give an idea of how much stuff there is. Theres on of the kanji books on the far left, and theres a second one that size too) Again, British publisher's textbooks I don't like very much so I've used the Minna no Nihongo (above) and Japanese for Busy people in the past. (Make sure its the kana version!!)
I was given these books by my 6th form Japanese teacher when she had a clear out and they are very useful! Basically they are about colloquialisms, contextual uses and everyday uses of the language. Not really reference books, but still interesting. The series is called Nihongo Notes. You can find the odd copy on Amazon second hand, otherwise I would try ebay or a import book store!
Now we move onto all the random reading material I have lying around XD
I have primary school reading books books and school level Kanji books too (in storage I think...). The reading books tend to be completely in kana, so are not very useful once you move onto kanji, but they're still cute ^.^
I also have a few books, novel and manga
You can also buy books with short stories in them with the English translation and language notes in them. This isn't exactly what I mean, but the above travel book has both the Japanese and English in it.
I also found this random book on Kimono!! XD
For buying books from Japanese publishers and books in Japanese in the UK I recommend JP Books in London. (No I am not getting paid to say this!) They do have a loyalty card thing where you can collect points and you can get a voucher once you've collected enough and if your studying Japanese at University they also do a slight discount if you have your student card (on Language learning stuff only). But, as all their books are imported, please expect as slightly hefty price tag! They also do the JLPT workbooks.
For magazines and manga theres also Japan Centre in London! Also its worth stalking ebay for everything I've mentioned above. A lot of my dictionaries are second hand and quite old, but they're better than what I could find in the bookshop or on amazon! One of my dictionaries I found in a local Oxfam for £2! For other second hand books, livejournal groups like Garagesalejapan are also worth a look. Sometimes Japanese language stuff crops up on there.
When I went to America I found that the book stores there tend to have a pretty good selection for Japanese dictionaries and stuff and I suspect the American Amazon has a better selection!
Remember when buying extra reading material, get something that interests you!! If you have any questions, or if you want to know if I've had specific experience of a book your thinking of buying, please just ask!! It can be hard buying books on the internet as you can't look inside! I hope this helps.