I don't think there is any sense in planning a second book for a few reasons. First off Heroes By Trade is focused on providing a system where they aren't endless charts and systems that are rarely used so the GM section is almost entirely advice in the form of text. That just isn't enough to fill a second book even if planning two books wasn't unrealistic from a practical standpoint. Secondly I tended to find that sequestering away that advice leads to a more adversarial situation where GMs are set up as the keeper of secrets rather than as collaborators.
Given that I want all of the GM section to fit into the regular book I have three ways to handle it. Firstly and most simply, I could just have a big ass chapter with lots of GMing advice in it. This has the advantage of making it crystal clear what is considered a rule the players should know and what is a design idea. The second option I have been considering is writing the GM advice right into the player's section. There are a few formatting options like having the page split vertically with GMing advice in the outer column and player advice on the inner column with a different background colour or simply having occasional whole pages devoted to GM advice and explanation. Lastly I could write the entire thing to both GMs and players alike and present everything as a set of options rather than a definitive ruleset.
I don't know which system would work better. Is it easier to have everything on a topic in a single location, or is having the GM advice there just going to confuse players and make the game harder to pick up? One of my major goals is to provide a simple format where a totally inexperienced player can figure out what to read and rapidly learn everything they have to know. I have even been considering special notation on paragraphs so that a new player could be told "read everything that has a star next to it" to get a really basic crash course.
Feel free to tell me which of my basic structures you think would be best given the goals I have outlined:
1. A GM advice chapter that contains subsections on Skills, Rituals, Combat, etc.
2. GM advice written in clearly separate blocks within the regular chapters.
3. The entire thing written without differentiating between basic rules and GM options and advice.