[Coen]

Check:
Straight forward overview (layout of lens etc)
http://members.shaw.ca/quadibloc/science/opt04.htm

Expanded:
http://www.astrosurf.com/re/evolution_of_eyepieces.pdf

General:
http://www.astro-tom.com/telescopes/eyepieces.htm

http://en.wikipedia.org/wiki/Eyepiece

Simulation of view: http://www.mydob.co.uk/eyepiece_simulator.php (does not really work for me, but might for you)

Try also: http://www.stargazing.net/naa/scopemath.htm