Here is a fairly complete list of worthwhile books (i.e., not scurrilous or hateful) about Justice Thomas, his biography, his jurisprudence, etc.
My Grandfather's Son: A Memoir, by Clarence Thomas. Looking forward to getting this one.
Judging Thomas: The Life and Times of Clarence Thomas, by Ken Foskett. Here's a review by Henry Carey in the Law & Politics Book Review. Here's another good review by John Eastman, a former Thomas clerk.
First Principles: The Jurisprudence of Justice Thomas, by Scott Gerber. Here's is a review by John Eastman, and here's a review by Lucas Morel.
Supreme Court Opinions of Clarence Thomas 1991-2006: A Conservative's Perspective, by Henry Mark Holzer.
Clarence Thomas: A Biography, by Andrew Peyton Thomas. This is the most thorough biography that you'll ever find; it traces Thomas's roots back to slavery days. Peyton Thomas is a Harvard Law graduate, and he dismantles Anita Hill's testimony better than anyone I've ever seen. Here's an interview with him. Here's a review by Ramesh Ponnuru in National Review.
Supreme Discomfort: The Divided Soul of Clarence Thomas, by Kevin Merida and Michael Fletcher. This is based on their 2002 Washington Post article (discussed here). Several articles drawn from the book appeared in the Washington Post in 2004 (see here). A lot of the book is reasonable, but I can't really recommend it, as (in my opinion) it dips too much into the scurrilous and takes Anita Hill far too seriously. Also, the chapter on Thomas's usual silence at oral argument struck me as unfair.