The problem is that the
tufte-book class is not yet able to work with LuaTeX > 0.85 which is a bit ridiculous, because it's been quite some time since that release.
tufte-book class checks whether it can use the
letterspace package by querying the value of
\pdftexversion. If that value is unset, it falls back to
soul. Modern LuaTeX does not have the builtin
\pdftexversion anymore (because it deviated quite substantially from pdfTeX over the years) and therefore the test fails and the incompatible
soul is loaded.
Luckily there is a compatibility package, which emulates the old interfaces and allows old packages to work with new LuaTeX. Just include it before