Although some of Diana Wynne Jones's stuff is interrelated, others are standalone. Homeward Bounders, Dogsbody, Tale of Time City, Aunt Maria, Archer's Goon, Time of the Ghost, Fire and Hemlock, Ogre Downstairs, Witch's Business, Wild Robert, Power of Three - all standalone.
Mine are all standalone, but I don't know how low you like your fantasy. I set mine in the real world with one fantasy element (at least, in all the stuff published so far). Dig in Time, Switching Well, and 11,000 Years Lost are time travel, Hobkin is fairies in West Texas, The Maze is a magic picture that sucks you in, Ghost Sitter is a ghost story, and the rest aren't fantasy.
Neal Shusterman is fantastic. Eyes of Kid Midas, Everlost, Fill Tilt, Dark Side of Nowhere - all standalone. Higher fantasy than mine, but not otherworldly.
High fantasy tends to be series-heavy because it's a certain amount of work to make a new, viable-seeming fantasy world (unless you're DWJ, who makes them like other people make sandwiches) and you don't want to start over from scratch. Also, readers want to explore more.