Derctuo is a book of notes on various topics, mostly science and engineering with some math, from the first year of the COVID-19 pandemic, 02020 CE. Its primary published form is a gzipped tarball of 9MB of HTML files and sources, although there’s also an inferior PDF version of about 1000 pages for reading on hand computers or printing. It uses a page size slightly smaller than standard for improved readability on hand computers. My original plan was to write a reproducible computation system so that the book would be entirely reproducible from a minimal computational core, allowing all of its calculations to be not only verified but also easily extended, reused, and studied. I didn’t get very far on that plan. Instead it’s mostly just about a quarter million words of dead text, with some static inline images, plus a bundled library of source material which is not included in the PDF version. It contains some novel discoveries, but some of it is just my notes from exploring the enormous feast of knowledge now available on the internet to anyone who takes the time to taste of it, and some other parts are explorations that didn’t pan out — left here only as a cautionary tale to the next explorer. There are lots of notes in here that aren’t “finished” in the usual sense; they end in the middle of a sentence, or say “XXX”, or have a note in them that the foregoing is wrong in such-and-such a way. But I am publishing the final version of Derctuo today. I might make future versions of some of these notes, but not of Derctuo itself. Derctuo is the sequel to Dercuano, <http://canonical.org/~kragen/dercuano> a larger collection of my notes which I published in 02019.