A big thanks to all people who replied to my mail, based on your 
recommendations I will be getting the following books from 
http://www.amazon.co.uk

"Unix Filesystems: Evolution, Design and Implementation" by Steve Pate
"The Design and Implementation of the FreeBSD Operating System" by 
Marshall Kirk McKusick
"TCP/IP Illustrated: The Implementation v.2" by W. Richard Stevens

There is probably a lot more good reading material which I will discover 
as I gain more experience in this area.