Master and Apprentice will take place before The Phantom Menace.
io9 posted a story detailing some future book and comic publishing coming out of the Star Wars universe, and for you PT fans out there (that includes me) there’s quite a bit to be excited about. There’s a book about Padme that takes place between The Phantom Menace and Attack Of The Clones by E.K. Johnston. I’m most excited about Master And Apprentice, a novel about Qui-gon and Obi-Wan being written by Claudia Gray. I’m a big fan of Ms. Gray’s Lost Stars and Bloodline novels; in fact I think they are some of the best work that has been produced in the Disney era.
This makes the "vote for Pade (sic)" campaign an unmitigated success.
In comic news, IDW will be publishing a new series called Tales From Vader’s Castle and the current Darth Vader series will be taking us to the Dark Lord’s crib in an arc called Fortress Vader. There are also plans for three separate anthology mini series taking place in three different areas: Age of Republic, Age of Rebellion, and Age of Resistance.
Editor's Note: I'm not so sure that Disney should be reinforcing that "Vader's Castle" is a thing.