The first Doctor Who book I ever read was written by Terrance Dicks and published in 1977 - Doctor Who and the Deadly Assassin! As usual, I picked this up at an antique bookstore. It only takes a couple hours to read (although I have been told I am a speedy reader - I usually read a novel a day) and gives great insight into the workings of the Time Lord society, and the history of Rassilon.
Fun fact: Not all Gallifreyans are Time Lords.
Even more fun fact: The book is based off of the four episodes that made up the "The Deadly Assassin" serial, which can be watched on youtube! Thanks, technology and copyright infringements!