First draft

Actually it's more of a first draft of a first draft. I'll keep updating the pull request until there's a proper first draft. The beginning, in particular, is quite choppy and decrepit. The middle isn't that great either. But the end is pretty good... right up until you fall off the page. 
Your chapter is lacking an early hook so far; however, it looks like you actually have a good one here. I would suggest:

a) starting your chapter with a specific problem statement (possibly this hobby connectivity problem)
b) explaining how graphs are a natural data structure for this, and maybe going through some of this code walkthrough (independent of any conversation of persistence)
c) explain how we quickly run into the problem of persisting the entire graph in memory, and maintaining that state across sessions
d) Possibly outlining a few ways you could use more 'conventional' persistence schemes (flatfiles, rdbms) and why those are annoying
e) Explaining what a graph database is, and then building them.

For all of this, I think it would be helpful to motivate up-front the kinds of queries + state changes you want to make of the model (independent of implementation), and possibly the sorts of runtime characteristics you'd like them to have.