For trekking in the popular areas (Everest, Annapurna, Langtang), it's pretty pointless to have a guide. A much better option is a porter-guide(s), guidebook and a map. It's much better, safer and more enjoyable to set your own itenerary.
If you are climbing "easy" peaks, then a Nepali guide service would usually be recommended and more ethical (keeps more money local). Spending locally is pretty important, IMHO.
If you are doing something technical (Ama Dablam or an 8000 meter peak for example), then a western guide service may be the way to go.
If you are going to the popular trekking areas, the following books describe about every step of the way on the main routes:http://www.amazon.com/Trekking-Everest- ... 559&sr=1-1http://www.amazon.com/Trekking-Annapurn ... 589&sr=1-2http://www.amazon.com/Trekking-Langtang ... 680&sr=1-1
They also cover many routes and side trips well off the beaten track. It's pretty hard to get lost, even off the beaten track using a map/book and even if you did, your porter (porter-guide) will know where to go or will at the very least be able to ask. On the main routes, it's almost impossible to get lost.