1.USDA Natural Resources Conservation Service http://plants.usda.gov/
This site provides a precise distribution map of each plant’s. Also, it offers detailed information of plant in a pdf format. This pdf presents not just simple facts but some further knowledge of each plant.
2. PNW Plants http://pnwplants.wsu.edu/
This is a site of Washington State University. It lists basic facts that is key to identify plants in a organized format. The web page is pretty clear and easy to read.
3. Perfect Plant, Perfect Place, Lancaster, R., DK Publishing, London
This book kind of focus on the application of plants. It tells a lot about the suitable used of plants as well as how to grow them well. Some informations like water use or soil type are really helpful.