Guide Books:
Some useful links:
Nepal Tourism Board
Nepal Mountanering Association
Kathmandu Environmental Education Project
Himalayan Rescue Association 
Trekking Agents Associations of Nepal
High altitude Medicine Books:
The High altitude medicine book By Andrew J Pollard &David R. MUrchard.
High altitude medicine and Physiology By: Michael p. ward, James s., John h. West
High altitude illness and wellness By, Charles Houston