At the moment, tbook is still an XSLT 1.0 application. Eventually it will be 2.0, though. Then, but only then it will support Saxon 7. Unfortunately, I see no possibility to support both Saxon 6.5.x and Saxon 7.x at the same time.
I don't support Xalan. I've never tried that, maybe it runs through without error messages, but Xalan is not able to create shell scripts, which would make things hopelessly awkward.
All of this is nothing for me to worry about, because Saxon 6.5.x is just so easy to install, even parallely with Saxon 7.
As soon as XSLT 2.0 is a finished specification, tbook will support it. Then you can use tbook with all XSLT processors. Well, all that support XSLT 2.0. But Xalan and Saxon will be surely among them.