Well some of the harder to get ones are on the BBC website as E-books.
http://www.bbc.co.uk/doctorwho/classic/ebooks/
Human Nature is an excellent read , Lungbarrow is good too , it`s a little long winded but it gives some of the reasons as to why the Doctor left Gallifrey in the first place.Although it answers them with even more questions.
The Dying Days is supposed to be good to but i`ve not got around to reading it.
But the best book i`ve ever read is this one.....
Alien Bodies By Lawrence Miles
On an island in the East Indies, in a lost city buried deep in the heart of the rainforest, agents of the most formidable powers in the galaxy are gathering. They have been invited there to bid for what could turn out to be the deadliest weapon ever created.
When the Doctor and Sam arrive in the city, the Time Lord soon realises they've walked into the middle of the strangest auction in history - and what's on sale to the highest bidder is something more horrifying than even the Doctor could have imagined, something that could change his life forever.
And just when it seems things can't get any worse, the Doctor finds out who else is on the guest list.
In fact all Lawrence Miles books are great.