Clif Flynt wrote the book on Tcl. He also helps organize the annual Tcl/Tk User's Conference, speaks at Open Source events and has taught Tcl/Tk around the world.
It includes over 150 new pages covering new features in 8.6. There are chapters describing TclOO, Tcl Modules, Sockets, Threading and much more.
The target for this book is an experience programmer who needs to pick up a new language quickly. Tcl's primary features are covered in depth. New features and tricks of the trade are described in detail. Finally, there is an entire chapter devoted to optimization and debugging techniques.