For example, The Texas Water Development Board has put together an absolutely bang-up document on rainwater harvesting for farmers who are so far off the grid that municipal water is a (Ouch, punny) pipe dream. Thanks to the folks at Metaefficient, there's a PDF copy available online.
Inside, you'll find instruction on everything from rooftop collection to storage tanks, to filtration for shower use, or even drinking. Other interesting tidbits include a water budget for indoor and landscape use, and over 15 pages of actual case studies from around Texas and the world, so you can check you own setup against something. For anyone already living off the grid, or considering it, this would be an indispensable guide for your water needs. :: Rain Harvesting in Texas