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 "">here.

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

* Brandon Coya and Brendan Fong, "">Corelations are the
prop for extraspecial commutative Frobenius monoids
Theory and Applications of Categories 32 (2017),
380–395. (Blog article "">here.)

* John Baez, Brandon Coya and Franciscus Rebro, "">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, "">A compositional framework for
passive linear networks
. (Blog article "">here.)

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