One of the joys of discovering a new country for us is the preparation in advance and the reading of guide books so that you have some sense of what to look out for.
There is a wealth of general books on Nepal such as the Rough Guide and the Lonely Planet Guide which give full information - though the political situation has changed quite dramatically in recent years and it's worth getting the most recent information from websites.
For trekking purposes Cicerone Press and Trailblazer Books both provide a range of guides for specific areas but there are numerous other guides available. For maps the Map Shop in Upton-upon-Severn provides an excellent service. An alternative, if you plan to spend time in Kathmandu, is to visit Pilgrim's Books or one of the other excellent book shops in the heart of Thamel - the Nepa Map ( 1:15 000 ) for Kathmandu and Patan City with even greater detail for Thamel is invaluable.