Dave Cullen, author of Columbine 

Profiles of the killers

These are the best profiles/biographies of Eric Harris and Dylan Klebold.

The first, by Lynn Bartels is truly a standout. Alan Prendergast provides a thorough, textured portrait.

These were based on information revealed the first few months, so they don't include insights from the tapes, journals, etc. But as far as backgrounds on the two boys, they are excellent.

The Jeffco report also does a nice concise summary of basic information: