How could I conditionally theme selected links to sub-pages in the book module?

I would appreciate advise about how to approach a DRUPAL implementation problem. I would like to have different sub-pages of a book themed differently, according to whether the corresponding node has selected properties. As a result, i would like to mimic (but modify) selected parts of the book module, but want to do that if possible without having to incorporate changes into the module itself (to avoid being overwritten when I upgrade to new versions of drupal). With the limited parts of Drupal that I've been exposed to for development purposes, I believe there must be an approach to this, but I can't seem to figure out exactly what that is. Can someone sketch out an approach for this, or point me to some developer tips, or an existing pattern, for how this might be done?

Thanks so much!