Dangerous subject to get me started on!!!!!
I only have a relatively modest collection - I have only just cracked the 500 mark - and in fact only recently decided to cut out buying some subjects, namely Indycar, and books by Christopher Hilton! Hasn't made much difference yet.....
Favourite Autobiography I have read would be "Starting Grid to Chequered Flag" by Paul Frere, best non-fiction book "To Finish First" by Phil Kerr (anybody that hasn't read this book should be shot!) - mine has an inscription from Emerson Fittipaldi in it, that's another story - best reference books, toss-up between my Autocourse annuals, "Grand Prix Who's Who", and Peter Higham's "Guinnes Guide to International Motor Racing".
As far as Marques go, toss-up between "The Works Escorts", Allan Scott's "TWR and Jaguar's XJS", and "Cosworth" (5th edition).
As far as Rallying goes, "A Boot Full of Right Arms" by Evan Green, although "Marathon in the Dust" by Innes Ireland comes really close!
Of course, my Bathurst and The Great Race annuals cover the Touring Cars sector, "Le Mans '59" by Stirling Moss covers Sports Cars, and as for art, well, I won't try and separate "Brockbank's Grand Prix", "Formula One" by Michael Turner, or "The Motorsport Art of Michael Turner.
Lastly (whew!) anyone who wants a good laugh needs Maurice O'Reilly's "A Series of Small Explosions", and "Journeys With Gelignite Jack" by Evan Green.
Of course there's another 450 books I should have mentioned.......