My answer: "Too many!"
(or better: "Bought too many - now they are gone...")
It took me years to stop falling into the book buying trap - "Ah - a new book with my lines, even a repertoire book - and look at this one, I certainly can use it sometime - and here they have some "golden oldies" for a real low price". Money running out of my hands - chess wisdom did not come...
Something happened - I bought the best two books which exist! (from J. Rowson Chess for Zebras, 7 deadly sins) - 1 idea of him burns in my mind: "You have to work out chess / variations for yourself - you have to sit down and work, only reading means consuming and there is no improvement"
What I did then? I made some "feng shui" with my books and gave away nearly all of them, only keeping about 30 of the best and I am only willing to buy a new one if I have the time to sit down and work on it - as long as I havent worked with it I dont buy a new one.
Sorry dear chesspublishers - you are promising the easy way with DW, SOS, Beating the... and similar things - but who really reads it and uses it? I spent the bucks for chesspublishing and stopped buying opening books - I still receive "NIC" cause I like to read some storys.
On my birthday and christmas I grant myself a chessbook - I know, my flesh is weak (Play the Semislav for birthday, hehe) - and now I feel free. I play chess, I analyse, I spent time on this forum (ugh?!), I play through the updates and you know what - I improve!!! - No more chess flooding rubbish - but quality.
I think Mr. Rowson deserves a "Chess Oscar" - dont read his books, some publishers may sell no more afterwards...