I’m hammering out the broad shape of two big subplots in my new book. Sadly, I don’t believe that any book will help me with this. Subplots are unique to each story. But some elements are standard for subplots that span the length of most the book.
Introduction. Characters may be set up in the Introduction or they may first appear in the Early Middle. Since major subplots typically do not start in the Introduction (common exception: major romance subplot), the characters and/or situation are often in stasis at this point, maybe feeling the pressures of the situation (or maybe not) but not yet pushed into doing anything about it.
Arc. Subplots are structured very like the main plot with a set-up, inciting incident, crisis that makes the character(s) turn from reactive to proactive, a point that propels them into the final showdown, the show-down, and resolution (optional). These points may or may not occur at the same time as in the main plot.
Timing. Subplots alternate with the main story. Long-term subplot are usually introduced in order of importance (the most important – like a romance – first). And finish up the least important subplots first. Subplots typically are resolved before the final battle of the main plot. A possible exception can be when the outcome of the subplot depends on the final battle’s result.