Because our IT sphere develops so fast it is so easy to wake up one morning and understand that your knowledge about things got outdated. I have personally decided to restart my habit of reading technical books. Lately I have completed book about Varnish (https://www.varnish-software.com/static/book/) that I highly recommend to everyone dealing with web development.
Beside of reminding different options in VCL configurations in vcl_recv I have rediscovered possibility of using varnish as load balancer (multiple backends), grace (https://www.varnish-cache.org/trac/wiki/VCLExampleGrace -- deliver cached content when backend fails to answer on health checks), ESI's.
One extremely useful feature (I wish I knew it half a year ago) is proxying requests to different domain using varnish backend setting. In this way we can hide CORS AJAX requests to different domain and in this way prevent hackers to try attacking that site. In one of our previous projects we have used delay writes to database. Our comments were submitted with AJAX to different servers (non drupal php application that were storing comment submissions to redis queue). The problem is that we wouldn't like spam attackers to approach that server. If I know about varnish functionality we would be able to hide from external world fact that we are actually using different server for collecting comments. Also it would be much more secure as we would filter on our comments server requests by IP address of our varnish server.
I would like to that the authors of the book that allow everyone to download PDF version of it. That is the best approach you can take for open source project.