There's a distinct tonal and genre shift between this and the first two books in the series. There's no romance, for a start - Rosie and Don are happily married, with no relationship problems between themselves to solve, and that relationship is a minor thread within the book, whereas in the first two it was the central pairing. I've no complaints there, further problems might have come across as repetitive, and the focus of this book is on their role as parents to Hudson, now ten years old. The other difference is that there's not a lot of humour, either. This just isn't funny in comparison to the first two, but honestly, I find that an improvement - this is my favourite of the three, I think. I always felt a bit, with the first two books, as if I was laughing at Don rather than with him, but the somewhat clownish presentation of a non-neurotypical person is absent here, and it's a shift for the better. Also, no one really wants to be laughing at a young kid struggling in school through no fault of his own and finding it difficult to make friends. 

Hudson, however, with help from his Dad - and Don is an excellent parent - finds his own way through, refusing to compromise who he is while learning new skills and making new friends with kids who both are and are not like himself. He's a very sympathetic kid, and Don becomes more sympathetic himself when interacting with him. It's nice to read books that focus on good parents, and both Don and Rosie absolutely qualify.