TromboneChat

Which Edwards tone builder book is best? I'm looking at The Intermediate trombonist but I know there are more.

That's a great book. Start with it

There is a great deal of material for maintaining and improving your playing generally in "Trombone Craft." "Bass Trombone Craft" is in a similar vein, with considerable specialization towards lower register and valve/position choice.

Start with Intermediate trombone. He has recommendations for next books in that book.