If you're interested in free Who, all of the novels once offered by BBC are available at
feedbooks or
manybooks. The total number is 5. (That is the ones offered as ebook downloads, not the ones that were available as html only.)
Also, if you are a fan of the Virgin-era Who books (which all of these were), the novel "Who Killed Kennedy" by David Bishop is available from the author
here in pdf format. It doesn't actually have the Doc, but it has many supporting characters from the show and does take place within the overall Who "universe."
Enjoy!
skinmaan