Yeah, this is on my list. So far, I've only designed the TOC down to
the chapter headings.
I've got some other code to finish up first, but if you'd like to take
a stab at the TOC design, look in the tufte-common.sty file toward the
bottom (search for "main TOC"); you'll want to add more \titlecontents
to format the other heading styles.
Alternatively, you can toss out the Tufte-style TOC for the time being
by passing the 'notoc' option to the documentclass:
\documentclass[notoc]{tufte-book}
This will ignore the Tufte-style TOC and just use LaTeX's default TOC
formatting.
--Kevin