There is one type of Japanese reference book that I often run across in bookstores that always makes me wonder. It’s the Japanese verb dictionary which have several hundreds of pages, each containing a verb and its various conjugations.
For a beginner where money is no object and you just need to double check your conjugations, it’s true that you could make some use of these. But frankly, I think these books are mostly a waste of money. Here’s why:
1) Once your computer is configured for Japanese input, you can type in your guess at a conjugation of a verb and if it is wrong you’ll usually know right away. For example if I type “たべった” and hit spacebar, I get “多べった” and the underline will be separate between the first and later characters, indicating they are detected as different words. Conversely, when I type “たべた” and hit space, I immediately get “食べた” with a solid underline.
2) Verb conjugation in Japanese is much easier than many other languages, such as English and Spanish. It’s relatively simple, you just need to memorize a few categories and subject doesn’t affect conjugation. There is also very few irregular verbs (at least in Tokyo dialect), with する and くる being two common examples.
The only thing which can be tedious to memorize is whether a verb is in the iru/eru category (食べる「たべる」、見る「みる」) or not (喋る「しゃべる」、要る「いる」), because this affects how the verbs are conjugated. However you can use #1 above to figure this out quickly.
3) The Wikipedia page on Japanese verb conjugation is very complete and easy to understand. Use that as a basic reference:
http://en.wikipedia.org/wiki/Japanese_verb_conjugation
My stance on this topic is similar to that of not learning hiragana right away, which I discussed here. The disadvantages of using one of these verb references, besides the fact they take up money and space, is that it may take you longer to memorize the various conjugations since you use it as a crutch.
If the verb dictionary has a useful section other than the verb conjugation charts, which take up a good portion of the book, then it still may be worth purchasing.
Note: I was planning on adding “use Google to search for your guess at conjugation” to this list, but when I tried it myself I got thousands of hits for “たべった” (incorrect), so on second thought don’t try that (: