Wolfgang C. Hellrigl:     The Classic Stamps of Nepal, 1984

Wolfgang C. Hellrigl:     Nepal Spezialkatalog, 1981

Wolfgang C. Hellrigl:     Katalog der Poststempel von Nepal (1879-1935), 1982

Hellrigl/Hepper              The Native Postmarks of Nepal

H.D.S. Haverbeck:        Postage Stamps of Nepal 1963

Postal Himal:                 Die Mitteilungshefte des “Nepal and Tibet Philatelic Study Circle”.

Indien Report:                Die Mitteilungshefte der Forschungsgemeinschaft Indien im BDPh.

Philatelic Organizations
The Nepal and Tibet Philatelic Study Circle, the place for collectors of the Stamps and Postal History of Nepal (Will be opened in a new browser window)

Miscellaneous
If you like to convert the Bikram Sambat (BS) dates as being used in Nepal Postmarks from the classic period into Gregorian Dates (AD), you might what to download the English-Nepali Date Converter. There are also online date converters on the Internet but they require a constant Internet Connection. The small utility program is a stand alone version (For Windows based computers). (Will be opened in a new browser window).
NOTE: 23rd. October 2010.
It seems the program is not avialabe for download anymore. If I can locate the file again I will add the new download link.