Release It!: Design and Deploy Production-Ready Software
If you can only read one book on running production systems, start with this one — a different angle on the same problem space, more enjoyable than the SRE book.
If I have to choose one book on production-ready software, I must say I enjoyed Release It! a bit more than the SRE book. They both are good and have a different angle on the problem, but I recommend starting with this one.