I am not sure how many there have been to date but I remember in 2004 a bookstore owner in Madrid told us that over 400 camino books had been published in the 2003 - 2004 period.
A search on Amazon using "camino" as a search word will give 23,461 results, but these will include Chevvy cars!
Santiago de Compostela as the search words, offers 4,914 results.
Camino de Santiago 798 results.
There are lots of camino books on sale at the moment - Joyce Rupp's from $4.21, Edwin Mullins from $2.62, Fumbling (which had a really good review when it was released) for $1, The Pilgrimage Road to Santiago: The Complete Cultural Handbook by David M. Gitlitz and Linda Kay Davidson $8,74 and so on.
The Road to Santiago - Walter Starkie in paperback is $77 but you can buy it through the CSJ in the UK for £8.99 ($17).
Just as a matter of interest, this is a January 1957 The New York Times book review of Walter Starkie's Santiago book:
"The road to Santiago, like many other books replete with curious information, is better savoured than gulped. Tasty in bits, it seems monotonous and dull in large doses."
I agree - it is a book that you can go back to over and over again for reference and find new items of interest, but read in one sitting it can be a bit overwhelming. (Much of the book is based on information gained from Georgiana Goddard King).