There is not a lot of fitness training information specific to squash available on the internet.
One excellent site is a blog entitled "The Science of Coaching Squash," located here. This site is presented by Tim Bacon, a fully certified squash coach and strength and conditioning specialist. Bacon is an experienced fitness guru, and well known in the squash world. Check out his often-entertaining site and learn the science behind coaching squash.
Another resource I've recently become aware of is the Fit and Functional website, found here.This site is run by Charles DeFrancesco, whose Fit and Functional company offers personal fitness training either at clients' homes or at various locations in New York and Connecticut. He also consults for schools and fitness centers. DeFrancesco is also certified at the highest level, and his stable of trainers all are well-certified and some have specific, squash-related experience.
But one of the greatest things about DeFrancesco's website is the free e-book on squash-specific training that is downloadable from his 'education' page. Click here for the book, which I heartily recommend, and get training. The book is 176 pages of information that can help you keep going while others whither and avoid annoying injuries. Check it out.