seL4 Reference Manual (WebArchive)
seL4 Reference Manual: Version 3.2.0 (my notes)
many more
From L3 to L4: What Have We Learnt in 20 Years of L4 Microkernels?
2009 Paper on Formal Proof
Comprehensive formal verification of an OS microkernel (my notes)
Formally Verified Software in the Real World
Red Team