Thanks, everyone! I urge you all to continue reading beyond Chapter 4.

Chapter 5 is about signal flow graphs in control theory, and "props", which are certain nice monoidal categories. My students Jason and Brandon wrote their theses on these topics:

* Jason Erbele, Categories in
Control: Applied PROPs
, Ph.D. thesis, U. C. Riverside, 2016.

* Brandon Coya, Circuits,
Bond Graphs, and Signal-Flow Diagrams: A Categorical Perspective
,
Ph.D. thesis, U. C. Riverside, 2018. (Blog article "https://johncarlosbaez.wordpress.com/2018/05/19/circuits-bond-graphs-and-signal-flow-diagrams/">here.

and Brendan has worked on these too, and so have I:

* Brandon Coya and Brendan Fong, "https://arxiv.org/abs/1601.02307">Corelations are the
prop for extraspecial commutative Frobenius monoids
,
Theory and Applications of Categories 32 (2017),
380–395. (Blog article "https://johncarlosbaez.wordpress.com/2016/02/02/corelations-in-network-theory/">here.)

* John Baez, Brandon Coya and Franciscus Rebro, "https://arxiv.org/abs/1707.08321">Props in network theory,
Theory and Applications of Categories 33 (2018), 727–783.
(Blog article here.)

so this material is almost painfully familiar to me.

Chapter 6 is on circuit diagrams, and both Brandon and Brendan did their theses with me on this topic. Brandon Coya's thesis is above, and Brendan's is here:

* Brendan Fong, The Algebra
of Open and Interconnected Systems
, Ph.D. thesis, University of
Oxford, 2016. (Blog article here.)

based in part on this paper, which is still fighting its way through the referees:

* John Baez and Brendan Fong, "https://arxiv.org/abs/1504.05625">A compositional framework for
passive linear networks
. (Blog article "https://johncarlosbaez.wordpress.com/2015/04/28/a-compositional-framework-for-passive-linear-networks/">here.)

Chapter 7 is on topos theory, which is very different, but also fascinating.