I do not think you will find a book devoted to the Hungarian Defense, but a surprisingly encouraging resource is Jan Pinski's "Italian Game and Evans Gambit," where you will find three or four wins by Black discussed with the Hungarian -- even a game with the exchange line 4..d6 5.dxe5 dxe5 6.Qxd8 Bxd8 where Black wins. I have not looked at these lines closely, since I have mainly been interested in lines for Black with an early ...g6 (e.g.: 1.e4 e5 2.Nf3 Nc6 3.Bc4 g6!?) which Pinski is also very positive about, and I think his analysis mostly holds up in that section, so overall he might not be so bad.
However, if you are interested in the type of positions typical of the Hungarian, you might be better off to consider instead the Antoshin Variation of Philidor's Defense (1.e4 e5 2.Nf3 d6 3.d4 exd4 4.Nxd4 Nf6 5.Nc3 Be7), which is very similar and very solid and has many GM and IM supporters with recent games and analysis. A great book is "The Philidor -- A Secret Weapon" by Christian Seel. The big advantage of the Philidor is it will cut down a lot on the theory you will need to know -- and you can also play something similar against the d-pawn and flank openings with an early ...d6, as described for example in "An Explosive Chess Opening Repertoire for Black" by Yrjola and Tella.
Also interesting is the ...g6 line I mentioned, which also allows you to sidestep the Two Knights or Giuoco Piano. I have two articles so far on it:
http://www.kenilworthchessclub.org/kenilworthian/2009/06/black-fianchetto-system...http://www.kenilworthchessclub.org/kenilworthian/2009/07/black-fianchetto-system...More to come. Here Pinski is best.
Finally, a third possibility worth considering as an alternative is the Closed Giuoco Piano line with an early ...Qe7 (e.g.: 1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.c3 and now either 4...Bb6 or 4...Qe7 followed by ...d6 and strong-pointing e5.) There are some classic games (see especially those of Euwe--and some encouraging games in Chernev's classic "Logical Chess, Move by Move") where Black does well with this. It seems a tad more active than the Hungarian but with a similar strong-point idea. You would then need lines against the Evans (perhaps declining with 4.b4 Bb6 or 4...d5!? or going for the Stoneware 4...Bxb4 5.c3 Bd6!?) and Max Lange Gambit (4.O-O d6! is best).