Definitely the best reference for all botanical information is
'The Citrus Industry, Volume 1' last printed in 1976 and now of-course out of print. One way of getting hold of a copy is to use the out-of-print book search facilities available on-line. I've successfully used:
Bibliofind  (now part of Amazon)    
Alibris
Abebooks
originates in Florida. It is orientated towards the commercial growers. Includes many more links as well as the forum.
My Citrus Books
Ultimate Citrus
REFERENCE BOOKS
PLANT  COLLECTIONS
There are 'citrus genetic resource' collections in
Valencia Spain
Riverside, California
The University of California
Corsica
These sites are really useful for investigating unusual varieties. The Corsican site also has numerous photos, a downloadable booklet of
5200 Citrus Names  and a description of
budding techniques.
WEB FORUMS
Limette Online  is a lively citrus discussion group - in German.
My Citrus Collection
Answers please
PLANT SOURCES
Citrus Nurseries
OTHER CITRUS LINKS
Australian native citrus excellent article on these species
Australian Native Produce Industry  downloadable factsheet on native citrus.
The Society for Growing Australian Plants has a map of Eremocitrus distribution.
Plants for a Future  covers Poncirus trifoliata, while
The-Banana.Com is a private site that has a section on citrus.
Florida Southern College has courses for students of the citrus industry
page updated 14 January 2002