The Missouri Botanical Garden web site http://www.mobot.org/gardeninghelp/ is an excellent source of information for gardeners who need information on garden, tree, or ornamentals culture/pest control. Unfortunately, their server is very slow and often down. There is also a telephone help line manned by volunteers and staff if you can't get thru to them via the web.
For grass, trees, shrubs, fruit, vegetables, and ornamentals the Missouri University Extension Service has an extensive list of publications available on their web site : http://muextension.missouri.edu/xplor/agguides/hort/
The University of Illinois has a similar site at http://web.extension.uiuc.edu/extension/hort.asp
Just about every other state land grant university in the US has a web site similar to the two above but geared more to their region. Over the years I have found the most useful to be: (in no particular order) Purdue, Cornell, Michigan State, Univ. California at Davis, Ohio State, Penn State, University of Maine, Iowa State, Nebraska, Kansas State, Clemson, Univ. Maryland, Rutgers (NJ), Univ. Mass, Virginia Tech, and the Univ. Wisconsin. Taken together, there is an immense wealth of information available for those who have an interest.
Don't forget that the world wide web is also an excellent source of information (depending upon your searching skills, of course). I never cease to be amazed at what pops up when I do a Goggle search!