The Institute of Youth Sports at Michigan State University is one of the best research based educational sites available. The mission of the Institute for the Study of Youth Sports is: “to provide leadership, scholarship and outreach that "transforms" the face of youth sports in ways that maximize the beneficial physical, psychological, and social effects of participation for children and youth while minimizing detrimental effects.” The Institute is directed by Dr. Dan Gould who has worked with the USOC, USTA and a number of other national sport governing bodies performing research and providing guidance on many areas related to sport psychology, parent education and youth sport. The Director of Coaching Education and Development is Dr. Larry Lauer, who is also on the advisory board of the ITPA and is a leader in the field of coaching education and parent education http://www.educ.msu.edu/ysi/forparents.htm
Responsible Sport Parenting is a great site that is sponsored by Liberty Mutual insurance, and provides some very useful information and resources for parents of youth athletes in all sports
The USTA has performed extensive work over the past decade on parent education research and education for the parent of the junior tennis player. Much of this information is provided on their website.
The LTA (Lawn Tennis Association) has a good parent education website with some useful information http://www.lta.org.uk/players-parents/Supporting-your-child/
Some general blogs and other websites that provide good information for tennis parents
The Tennis Parent Bible http://www.thetennisparentsbible.com/
Parenting Aces http://parentingaces.com/
Sport Parent Support http://sportparentsupport.com