‘No offense to you, AW!M, but your encryption is much much much easier to break than any known encryption algorithm ...’
I agree, but a little ‘old school’ mixed in with whatever a person has might not hurt. Especially when you get clever, such as adding a ‘blackhorse’ code in the mix and private understandings with your contacts.
Some of Rommel’s personal spies once used a classic book as a cypher. It was only worked out when British found copies of the books in the packs of agents they caught.
It's true that the ultimate security, one time pad, could be done with a book like that. But the problem is the key distribution channel, that book or some other method. PKI solves that problem by using a public key that is publicized although it must still be authenticated. I don't think anything better than PKI has been invented for key distribution but there are different implementations like RSA and ECC that many have relative strengths.