It's certainly a gentler introduction than the current, 2,000-page z/Arch Principles!
And along similar lines, it's kind of fun, if not at all useful in any economically rational sense of the term, to write nontrivial source- and binary-compatible code that builds and runs on everything from a 40-year-old 370 to a current-model Z (speaking hypothetically, as I don't actually have access to even a reasonable facsimile of the latter; I have, however, tested as far forward as late-model OS/390 running in 64-bit z/Arch mode on emulated hardware).
And along similar lines, it's kind of fun, if not at all useful in any economically rational sense of the term, to write nontrivial source- and binary-compatible code that builds and runs on everything from a 40-year-old 370 to a current-model Z (speaking hypothetically, as I don't actually have access to even a reasonable facsimile of the latter; I have, however, tested as far forward as late-model OS/390 running in 64-bit z/Arch mode on emulated hardware).