Watertown was recorded in 1969 and is arguably Sinatra's least successful album. Written entirely by Jake Holmes and Bob Gaudio, Watertown plays more like one long cohesive piece rather than a collection of songs. Technically, it would be a concept album, but I think it goes beyond the normal range of a concept album.
I stumbled upon this album in a thrift shop and picked it up knowing only that it was a Sinatra album. I loved it as soon as I started listening though, and it didn't take me long to realize this was something different. It has quickly become one of my favorite albums.