For a long time, the only Doctor Who books you could get were the Target novelisations of the tv episodes. Then, way back in 1991, after the tv show had been cancelled, Virgin Publishing bought the licence to publish Doctor Who stories, and started publishing their line of New Adventures, featuring the seventh Doctor.
Originally marketed as featuring stories that were "too broad and deep for the small screen", the first few books didn't really live up to that claim, being pretty standard Doctor Who stories with added nudity. Then came Timewyrm: Revelation, Cat's Cradle: Time's Crucible and Cat's Cradle: Warhead, three stories in a row that, for different reasons, could only really have worked as books.
From there, the line developed and, while there were always going to be books that were little more than a standard sci-fi adventure story with a generic monster of the week (like Shadowmind), there were a great many remarkable and creative stories in the series (like Human Nature and The Left-Handed Hummingbird). New companians were created, and the existing characters of the seventh Doctor and Ace were developed far more than they ever were in the original series.
Virgin later started publishing the Missing Adventures, featuring the other six Doctors and slotting in between the televised stories. These were generally more traditional than the New Adventures, with more limited scope for character development. This series still contained some gems, that managed to add to expand on the original series rather than merely mimc it (for example, The Dark Path gave us the story of why the Master turned evil).
When the Doctor Who tv movie was made in 1996, the BBC saw a chance to make some money from the book franchise, and took over the licence for Doctor Who books themselves. They published stories featuring the eighth Doctor and stories featuring earlier Doctors simultaneously, with no particular distinction between them. They became known among fans as the Eighth Doctor adventures and the Past Doctor adventures.
The Eighth Doctor adventures struggled at first to properly capture the character of the eighth Doctor, having only an hour or so of screen time to work off, and also suffered from a poorly conceived companion (the more-politically-correct-than-thou Sam Jones). Later editors for the range gave it a more consistent feel and direction, and it now (in my opinion) surpasses the New Adventures. Recent developments have seen Gallifrey destroyed, the Doctor stranded on Earth for a hundred years with no memory and the loss of the Doctor's second heart.
BBC's Past Doctor adventures faced similar problems to Virgin's Missing Adventures, but the BBC have allowed more of the past Doctor adventures to be more experimental in style and substance (like the first person non-linear narrative of Eye of Heaven) and have produced some first rate books (The Witch Hunters and The Festival of Death, for example).
I have started noding the books, starting with the New Adventures, which I have recently been rereading. I'll also start noding some of the BBC books I've read recently.
So far, I've done: