The older books which are still in publication seem to all be 8.5 or 8.6 and are relevant, I don't recall coming across anything which did not work yet but I am not exactly working through them cover to cover, used them to get the basics and am now using them more as a reference than a guide. Might not be the latest and greatest methods but will get you going. Probably nearing end of life since 9 is in beta, hopefully they will get updated.
8.6.14 released February of this year, with 9.x beta May 2024.
Tcler's Wiki has recent posts:
https://wiki.tcl-lang.org/
Still hoping the older books are relevant.