You’ll be able to get most or all of your questions answered by your rafting company, assuming you’re going on an organized trip.
Independent rafters and kayakers, however, should get hold of a copy of White Water Nepal by Peter Knowles and Darren Clarkson-King (Rivers Publishing, 3rd edition Jan 2012); it provides comprehensive river descriptions (including detail on a host of little-rafted rivers not mentioned here), maps, and excellent advice on logistics.
For more information, updates and links to operators, visit w raftnepal.org or the website of the Nepal River Conservation Trust (w nepalrivers.org.np). Himalayan Map House publishes rafting maps for the more popular rivers (such as the Sun Koshi and Trisuli), showing rapids, put-in points and so on. For other areas, trekking maps can keep you oriented.