If you want to read about and learn about all things related to plants and gardens, the Weekend Gardener is one of the best places on the Web to go.
This is an on-line gardening magazine and every month they add fresh new articles and informative "How To" tutorials. The information is always current and up to date, and there is an excellent archive. And it's free!
This site is written by garden industry professionals, and is for any level of gardener, so you should be able to find all the answers you need.
Sign up, and they will e-mail you each new addition of the "magazine."
Photo credit: Weekend Gardener