The book is finally out. You can buy it in soft or hard cover, kindle and other ebook formats.
You can find it on the popular e-stores or at the publisher: https://www.routledge.com/Developing-Virtual-Synthesizers-with-VCV-Rack/Gabrielli/p/book/9780367077730
The source code for the examples is available at my github repo: https://github.com/LOGUNIVPM/VCVBook/
My webpage will keep you updated with more material and news: https://www.leonardo-gabrielli.info/vcv-book