Both of Robert Thompson's book's, Close-up on Insects (ISBN 1-86108-238-X) and Close-up & Macro (ISBN 0-7153-1903-5) both extensively cover the subject of macro flash photography. Robert's latest book Close-up & Macro is more up-to-date covering digital, however both are really worth reading.
Go with a 'off the camera' flash gun, rather than a ringflash, as ringflashes tend to provide shadowless lighting making pictures look flat and low in texture and detail .
Nikon flashguns are generally excellent, however Metz maywell offer you a slightly cheaper alternative and have an excellent website,
Metz - always first class. -
Warehouse Express sell a nice flash bracket the Kirk FB-8 for just under £50, you'll find it under tripods if you wish to take a look, is well made and offers near perfect adjustment when using a single flash gun, a Stofen diffuser is also an essential purchase.
Mark