Update free-programming-books-subjects.md (#11378)

Adding a PDF that introduces the world of logic, sets, and relations using Z notation.

Author: margaret-hu

books/free-programming-books-subjects.md

@@ -660,6 +660,7 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 * [Small Memory Software](https://smallmemory.charlesweir.com/book.html) - Charles Weir, James Noble (HTML)
 * [Software Technical Writing: A Guidebook](https://jamesg.blog/book.pdf) - James Gallagher (PDF)
 * [The Web Development Glossary](https://github.com/frontenddogma/web-development-glossary) – Jens Oliver Meiert
+* [Using Z Specification, Refinement, and Proof](http://www.usingz.com/usingz.pdf) -  Jim Woodcock, Jim Davies (PDF)
 * [Web Almanac](https://almanac.httparchive.org/static/pdfs/web_almanac_2019_en.pdf) (PDF)
 * [Writing Native Mobile Apps in a Functional Language Succinctly](https://www.syncfusion.com/ebooks/writing_native_mobile_apps_in_a_functional_language_succinctly) - Vassili Kaplan