"I'm the Doctor, and you're in the biggest Library in the Universe. Look me up."
The Doctor, "Forest of the Dead"
How many books have been written about the Doctor? There is no simple answer to that question. There are hundreds of biographies about each of his incarnations. Other books focus on his various roles (cf. Time's Champion, Defender of Earth, the Oncoming Storm, etc.). A comprehensive bibliography of Doctoriana would have to include biographies of his many Companions and others whose lives intersected his timeline. The Library is fortunate in possessing one of the few privately-printed copies of the autobiography of Harriet Jones; Jehil Mhrblxx's acclaimed Lethbridge-Stewart: A Life of Duty, as well as Adam Mitchell's controversial The Doctor and Me: Twenty Hours in the TARDIS. Add in histories of the worlds he has saved and/or destroyed, textbooks of temporal mechanics, and extracts from the Torchwood archives, and the total is somewhere in the tens of thousands. (n.b. This does not include books in the Library's Gallifreyana section; as no one has succeeded in translating them, it is unknown how many of these rare volumes mention the Doctor.)
It is entirely beyond the scope of this guide to list derivative works based on the Doctor's lives and deeds, such as Find Your Inner Companion, A Better Way to Live: Spiritual Secrets of The Lonely God, The Jelly Baby Diet, and Women Who Run With the Time Lord (Oprah Book Club selection for June 2017). Likewise, we will not attempt to include the vast body of fiction, drama, and poetry that has been generated over the millennia. These range from David Russell's lyrical Still Point of the Turning World (Man Booker Intergalactic Prize of 3578) to imagined sexual escapades of such frequency, duration, and complexity that even a Time Lord might find his hearts failing under the strain.
A sacred text from the Doctor's favourite planet contains the following warning: "Of making many books there is no end, and much study wearies the body." It would take decades to read and absorb all of the most relevant material about the Doctor; nevertheless, a legion of scholars have made it their life-work to do so. They have identified the core titles which are required reading for anyone who wishes to have more than a superficial understanding of the Last of the Time Lords (see Appendix 42).
For those who find even that short list daunting, I can find no better encouragement than the following words from Professor Stephen Harris, current Harkness Scholar at the University of New Oxford:
"The Doctor said, 'All sentient life forms are our kith'. The rest is commentary; now go and study".
From the preface to An Annotated Bibliography of Doctoriana, R.T.D. Lambert, ed., (New Oxford: University of New Oxford Press, 8746)