Unfortunately, I have to agree that there does not seem to be any Web sites out there that deal with aircraft systems as a topic.
To add to what warburton1 posted, a little more information on some books on the subject:
The Jet Engine (Rolls-Royce) ISBN 0 902121 2 35
As the title suggests, this book deals exclusively with powerplant structures & systems. Very informative about what it covers though.
Aircraft Structures and Systems (R. Wilkinson / Longman) ISBN 0 582 23656 8
VERY basic introduction to the subject, though useful if you're just starting out.
Aircraft Systems (I. Moir / Longman) ISBN 0 582 07223 9
Gives a good feel for what the a/c systems are used for and why the various types are chosen.
Aircraft Electrical Systems (E. Pallet / Longman) ISBN 007 155 2650
Not one I've read, but recommended by a licenced engineer.
With the possible exception of the last book none of the above are particularly technical (i.e. maths-based), dealing instead descriptively with what the various systems do and some basic design requirements.
Hope this helps