Might be a little bit of shameless self promotion but I wrote The Kaiju Survival Guide about a year ago. If you enjoyed World War Z (the book, not the movie) and the Zombie Survival Guide then it might be right up your alley. I am currently working on the other books in the series.
Other then that, you can’t go wrong with Jeremy Robinson. I think there is also a big book of Kaiju short stories on amazon somewhere.
This is the book I have. It has very mixed reviews; the important thing to note going in to this one is that it's not an in-depth reference book or comprehensive history. It's a fluff book. There's some interesting stuff, but it's a very light read.
There are plenty of other books on the subject though.
Monsters Are Attacking Tokyo is supposed to be very good.