Books

Understanding the Linux Kernel (Daniel P. Bovet, Marco Cesati Ph.D., ISBN-10: 0596005652)

Linux Kernel Development (Robert Love, ISBN-10: 0672327201)

Understanding the Linux Virtual Memory Manager (Mel Gorman, ISBN-10: 0131453483)

Writing Linux Device Drivers (Dr Jerry Cooperstein, ISBN-10: 1448672384)

Websites

LXR linux – Searchable cross referenced kernel source.