It's been well over a decade since I read this series, but I still remember that basic plot outline without having to "google" it. That says alot for L.J. Smith's skills as a storyteller. I was heavily into the teen horror genre at this time and read quite a number of R.L. Stine and Christopher Pike books, but if you asked me now what they were about -- I'd have no clue.
The Forbidden Game trilogy may be out of print, but you can still view the fabulous cover art at this fansite: LJS Cover Resource. Thanks to this site, I've been able to make some cool icons featuring Jenny & Julian from TFG as well as Elena and "the crow" from L.J.Smith's other fabulous series, The Vampire Diaires.
The Forbidden Game