Jorg Flum and Martin Grohe just published their new book titled Parameterized Complexity Theory. Flum and Grohe are two giants in finite model theory who have contributed so much towards the development of parameterized complexity theory, especially logical characterizations of parameterized complexity classes. At this moment, one can only purchase the book from Springer as they just appeared some time this week.
I can't compare this book with Downey and Fellow's book as I haven't read Flum and Grohe's. However, I heard from several people who proof-read the book before publications that Flum and Grohe's book is much more well-written and readable than Downey and Fellow's.