Bike shops in Kathmandu and Pokhara have up-to-date information on trails and roads, though of course they’re in business mainly to sell tours, and won’t divulge all their secret routes.
There is no dedicated mountain-biking guidebook, but James Giambrione’s Kathmandu Valley Bikes & Hikes gives reasonable if dated coverage of this area. A map is crucial, but shouldn’t be relied upon absolutely – and maps go out of date fast in Nepal. Nepamaps does a 1:50,000 scale Biking Around Kathmandu Valley map, and a 1:75,000 Biking Around Annapurna map, both available in Kathmandu. Coverage of the former extends beyond the valley into the Central Hills, and bike trails are marked, if not always entirely accurately; at the time of writing, the road to Jomosom wasn’t even marked, four years on. Otherwise, you’ll have to rely on trekking maps.