Hi Buttercup, I have a book I can send you, the best one apparently, called Introduction to Statistical Mediation Analysis. It is very clear and simple - check it out on Amazon. I would be happy to e-mail it to you, if you PM me your e-mail address. Or I can upload it for you - whichever is better for you. There are also macros that will allow you to use SPSS to perform a longitudinal mediation analysis. You can get the macros from this site: http://www.psychwiki.com/wiki/Mediation
Hope this is of help.(up)