Each line in this key was formed of a triplet that referred to the page, line and word-number of a specific word, which, when looked up (using the exact correct book), was in fact the relevant ‘recovery word’. In 12 lines, he’d noted all 12 of the words in his recovery phrase without actually giving anything away to a casual reader/attacker. So long as the attacker didn’t know the book it referred to, the recovery phrase would be secure.
A more elegant solution would be to make this a brain wallet. Either via a "traditional" brainwallet, or by converting the 12 words to a xprivkey in a non-standard (nor BIP39 compliant) way.
This will allow you to generate the words in a way that is actually random. Even if you were to use software that uses flawed RNG, it will not be as big of a problem because you are using a book that your device will never know about.
With the first solution (book), you are now reliant on hiding an entire book rather just a single piece of paper. If you lose your copy of the book, there is no guarantee you would find the exact same edition again and your coins will be lost. So that's a more difficult back up process and a single point of failure.
This particular solution has issues similar to that of using a brain wallet. For example, if the specific book is not written down anywhere, you may forget which book you are using, or your next of kin will have no way of recovering your coin when you die.
You can mitigate your chances of losing access to the particular version of the book by choosing a book that will likely continue being published, like the bible.