What do you mean by "modern"? Here are some relatively recent books covering since about 1990.
Modern GT Racing - Dave Friedman 2007
Inside IMSA's Legendary GTP Race Cars - Martin & Fuller 2008
Anatomy And Development Of The Sports Prototype Racing Car (The) - Bamsey 1991
GT International - The Cars - Die Autos - Les Voitures 1993-1998 - Cole, Hurel & Tons -1999
GT Endurance Yearbooks 1995, 1996 and 1997
Prototypes - The History Of The IMSA GTP Series - Martin & Wells 2000 - very expensive when you can find it
World Sportscar Racing 1989-90-91-92-93-94
Porsche Sport 1994-2013 if you like Porsches
RS Spyder - de Boer, Cotton, Hinz & Upietz
24 Stunden Nürburgring Nordschleife Yearbooks 2001-2013
FIA GT Yearbooks 2001-2009
Le Mans Series Yearbooks 2004-2011
Ferrari 333 SP - Franzini - 1998
Sports Racing World Cup Official Book 1999-2000
American Le Mans Series Yearbook 1999, 2000, 2007, 2008
Grand American Road Racing Yearbook 2000-2004
Racing With A Difference - The history of IMSA - Starkey & Bishop -2001