Actually, Downtime was a video spin-off originally. The book came later.
The first one ever was Wartime, about Sergeant Benton on a mission when he meets the ghost of his father or something. (It's been a while.)
Auton was a trilogy of stories with UNIT fighting the Autons.
Mindgame and its follow-up, Mindgame Trilogy are about a human soldier, a Sontaran and a Draconian who get imprisoned together during a space war. The soldier is Sophie Aldred, so you're obviously supposed to think she's Ace.
Then there's a thing called Cyberon, which isn't really related to Doctor Who, but has a sequel called Zygon which clearly is.
There's also a comedy thing with Sylvester McCoy not quite playing the Doctor - called "Do You Have a Licence to Save this Planet?"
Daemos Rising is a sort of sequel to Downtime, as well as featuring the Daemons, and also ties in to the Time Hunter books.
I think that's all of them.