Perennial favorite. The Book Barn in Niantic, CT was the first used book store I found when I first moved here in 2009. Now with four locations in Niantic, it has a huge selection of books on all subjects. The main location is also known for the cats of varying degrees of friendliness that roam inside and out. If you can grab one of the comfy chairs upstairs it won’t be long before one of the residents will find you. Just make sure they approach you rather than the other way around – signs warn that one or two of the residents are a little on the grouchy side.
Update July 2015: uploaded a few more photos of the main location.