Nepal is an increasingly popular destination and there are lots of books to tempt you, picture books, travelogues, trekking guides, etc. I checked lots out in the local library and bookshops whilst gathering information. I did purchase the following in 1999 before I went off to Nepal :-
These are both excellent books, the McGuinness one is more up to date and has some very useful hand drawn maps of the trek routes. You can buy both in Kathmandu in the many bookshops in Thamel and at Namche.
I also bought the Schneider map of the Khumbu (1:50000) from the The Map Shop at Upton-on-Severn, which is just a few miles down the road from where I live. This is a great map to have at home but easily damaged on the trail. This and many other trekking maps are also available in Kathmandu and along the trail in the Khumbu.
I also photocopied information on Kathmandu, Patan and Bhaktapur from travel books borrowed from my local library.