Both Up the Downstair and The Sky Moves Sideways are fantastic albums, and if you don't have them, then I can't really recommend them enough. Some people don't even have Signify, which is a shame. :/
Even On the Sunday of Life is worth picking up, it's more of a "fun" listen and interesting to hear, but Radioactive Toy, Nostalgia Factory and Nine Cats are all solid songs.