Some more advanced topics (like how to rebuild FPC from scratch) are covered in the so called buildfaq:

http://www.stack.nl/~marcov/buildfaq.pdf

Some notes about making porting Object Pascal code is at

http://www.stack.nl/~marcov/porting.pdf

Notes about why the 1.9.x/2.x Unix rtl deviates from 1.0.x and/or Kylix are at

http://www.stack.nl/~marcov/unixrtl.pdf