Let me just throw it out there before I get to my reviews - this is one book where I just recommend you see the movie. It had the perfect combination of believeable humor, stolen kisses, and plot to keep me entertained and I'm already planning to go see it again.
Something Borrowed was okay, but for once in my life I actually preferred the movie. The book is about two best friends - Rachel and Darcy - who have been best friends forever. The friendship is dominated by competition, with Darcy always winning. Darcy is more beautiful, more glamorous, more wealthy, etc. Rachel is her dutiful and loyal sidekick, the straight A, good-girl who never does anything wrong. Until she sleeps with Darcy's fiancee, Dex, on the night of her 30th birthday. And doesn't feel bad about it. The novel is told from Rachel's point of view as she learns to stick up for herself and not let Darcy win for the first time i her life.
Maybe I would've felt different if I'd read the book before the movie, but I feel that the movie portrayed the relationship between Rachel and Dex in a different light than the book did. The movie built up more of a pre-Darcy relationship between Rachel and Dex, which made me more sympathetic with what she'd done. In the book, the birthday incident just happens, and it's not quite as forgiveable when it doesn't come with the build up of a pre-Darcy law school crush.
Then there's Something Blue, which is the sequel to Something Borrowed, and follows Darcy on her journey after the whole Rachel/Dex fiasco. I'm not going to give many details so I don't spoil anything for you, but I actually kind of hated the book. This again might be the fault of seeing the movie and falling in love with the characters in the movie, but the story in Something Blue was just unrealistic to me. I may just be a cynical person, but I couldn't get over how Darcy could just change her life and have such a perfect ending after the book (and movie actually) forced me into hating her.
So to sum it all up, I would skip the books and go see the movie.