Hi Matt,
Sportscar racing always seems to be overshadowed by Formula One when it comes to the 'history' books. I guess it doesn't help that where F1 has a clear line of development from 1950 (or earlier, if you call it Grand Prix racing!), the history of sportscar racing is much more fragmented and inconsistent. So, maybe we have to specialise a little in order to find what we want!
From my collection, I can certainly recommend 'Inside Imsa's Legendary GTP Race Cars' by Martin and Fuller. The emphasis is certainly on the design and development of the cars, with plenty of photos of cars in-build as well as on-track. There's plenty of text, too, and although it's not an out and out 'history of GTP' book, there's enough to give you a flavour of what was going on.
'Anatomy and Development of the Sports Prototype Racing Car' by Ian Bamsey is heavier on the text and has more of a Group C bias, but is still worth tracking down if you want to know about the design and technology involved.
'Sports Car Racing In Camera 1970 - 1979' by Paul Parker is obviously much more photo-orientated than the previous two books, but certainly covers a wide variety of makes and models - the big 5-litre Porsche and Ferraris from 1970 and 1971, the smaller open prototypes in 72 - 74, Group 5 and Group 6 from the mid-70's, and by the end of the decade you've got all sorts of weird things going on before Group C was introduced in 1982.
Finally, see if you can get a look at copies of 'Spyders & Silhouettes' and 'Monocoques & Ground Effects' by Janos Wimpffen. Between them, these two volumes cover sportscar racing in Europe and the US from 1972 though to 1992 (there are also two earlier volumes - 'Open Roads & Front Engines' and 'Winged Sports Cars & Enduring Innovation', but I don't have these). They're not cheap, and they'll certainly put a strain on your bookcase, but they'll tell you who did what and when, with plenty of photos.
Beyond these, it's probably a case of finding books that are more event or marque-sepcific, depending on where your interest lies. For me it's Porsche, so that opens up plenty of options! I doubt you'll find a single volume that covers everything in detail from the Bentley Boys and D-Type Jaguars through to the modern LMP and GT's, though, it's just too wide a subject. Probably the best way to cover this range would be some of the Le Mans books - maybe 'Le Mans 24 Hours' by Brian Laban, or 'The British at Le Mans' by Ian Wagstaff'. Have a look on the Chaters website, or Collectors Carbooks - there might be something out there!
Best of luck!
SB