My advice to birders who do not speak Japanese is to try to get hold of the out-of-date and out-of-print, but available secondhand 'A Field Guide to the Birds of Japan' (FGBJ) illustrated by Takano that Ed Keeble mentions above in this thread. In my opinion, it will enhance your trip, and especially your planning. If possible, get the Mark Brazil guide (Birds of East Asia - BEA) as well, and also a photo guide such as the one Hokkaido Stu links to. If you are only going to Hokkaido, may I recommend '
The Wild Birds of Hokkaido' which is a photo guide in Japanese. The pages have English and binomial names, but the index has binomial only. It gives stars for rarity, and coloured dots for habitat, but unfortunately the seasonal information is only in the Japanese text.
If you have all three, you can use FGBJ for an overview and planning, BEA for detailed information, and the Hokkaido WBH photos, so you can see what's around in Hokkaido specifically.
Why do I recommend this out-of-date FGBJ despite the fact that it has old names for birds which have been split or re-named, and whose pictures (paintings) have come in for criticism from some sources, and where some of the distribution information may be out of date? What's wrong with the much more up-to-date 'Birds of East Asia'?
The main point is that the Takano book (FGBJ) contains Japanese birds and only Japanese birds (including accidentals and extinct species) while Brazil's book (BEA) covers a far larger area.
About half of the birds in BEA have never been seen in Japan (Brazil's book has almost exactly 1,000 species, and the FGBJ about 500). In addition, it is sometimes the case that the sub-species which occurs in Japan is not illustrated in BEA.
Thus with BEA you have to search through for your Japanese birds, and your bird may not be illustrated anyway, so you have to read through a long description of sometimes numerous continental sub-species to find out what the bird in Japan is supposed to look like. FGBJ, because it only covers Japan and Japanese sub-species, has more room for alternative poses and views. In my opinion, from a practical use point of view, FGBJ has better illustrations (I seem to be in a minority on this, however; but the BEA illustrations certainly vary in quality). Also FGBJ has identification pointers on the illustrations; BEA does not.
Furthermore, Japan is long and narrow, and so many species only occur in a limited area. Therefore it might be the case that only 10-20% of the birds in BEA will be relevant to your trip to Hokkaido at a specific season, while 25-50% of FGBJ may be relevant.
In addition, looking through FGBJ will give you an overview of what you may see (it may encourage you to expand your trip beyond Hokkaido). With BEA, far too many of the birds you see when flicking through and say, 'That looks interesting', will turn out not to be Japanese at all. It's dispiriting (in my opinion, anyway)
Another significant point is that FGBJ gives the birds' English names (sometimes changed now, due to splitting) but also the Japanese names in roman script. (BEA is purely English.) This means you can have some contact with local birders you meet maybe over dinner at a Japanese inn. I think it's fun to learn some local names, even if you are going around with an English-speaking guide. FGBJ is still in print in Japanese, and most Japanese birders are familiar with it, so if you are with a group and see a bird, they can often find it in the English FGBJ, to help you indentify it, even if they don't speak English, which they can't do with BEA.
Another thing is that the maps in FGBJ are 2.5cm square, and those in BEA are 2.2cm on a side, but BEA tries to fit in more information about seasonal location over a wider geographical area, and so many of the BEA maps need a magnifying glass to see what they are about.
Anyway, I think even think the out-of-dated-ness of FGBJ can be a good thing for you if you can afford the other books as well. If you use FGBJ as a base and overview, and then annotate the book with up-to-date names (about 5% of species, maybe) using BEA (or by posting here and asking users what they have changed), or looking around on the internet, and also maybe Hokkaido specific information, then you will have learnt a lot about the birds you will see in the process.
The thing a lot of English birders in Japan would like is for Mark Brazil's publisher to select the Japanese birds, get Mark to trim his description so it is Japan-centred, and publish a Japan bird guide (and the same for Korea). Although BEA is a wonderful achievement, it falls between several stools in its current form. For Japan, Korea, and Taiwan, it's a poor guidebook in the sense that a lot of stuff is not relevant. For China it has a relevance problem (our Japanese summer birds don't winter there, so a lot of the book is irrelevant to them also) and it cuts out arbitrarily in the middle of China. Despite what the cover declares, it covers only a part of China and pacific coastal Russia.