Dellorto tech book by Bob Tomlinson, the US dealer for Dellorto carburetors.
This book is mainly aimed at the DRLA carbs but has small sections about the DHLA and FRD carbs as well.
Topics covered in 125 pages:
Fitting, tuning, overhauling, jet selection & charts, troubleshooting, ignition timing, air filters, engine tuning etc.
This is the most comprehensive manual available for fitting and tuning Dellorto carburetors on VW engines.