Three Bird Summer by Sara St. Antoine is my newest read with this premise.
Adam and his family spend each summer at Three Bird Lake in Minnesota with his grandmother. This year is the first since Adam's parents got divorced, so his father isn't coming with them, and neither are his aunt or uncle or cousins. Adam's mom is worried about his grandma, who seems forgetful at times, and plans to stay at the lake long after Adam and his mom leave to go back to school.
The summer is looking pretty boring, until Adam meets Alice, who is living in the cabin next door. Alice is blond and pretty, and Adam is sure the two won't be friends. Yet, the two do begin to hang out, and it is Alice that Adam looks to for advice when Gram starts leaving Adam bizarre notes in his bedroom.
St. Antoine has created a good realistic fiction novel for both tween boys and girls, a perfect summer (or fall or spring or winter) read.
If you're looking for other books that fall into the "summer at the cabin" setting, here are a few more: