Funny, I just finished that book.

They were using a few more mappings per number for instance:

0: 's', 'z', soft 'c'

1: 't', 'd', 'th'

6: 'j', 'sh', 'ch', soft 'g'

etc

The problem I have, is that they present them from 1 to 0, and the first 4 have some mnemonic logic added to them: a bar in 't' for 1, two bars down in the 'n' for 2, three in 'm' for 3, and the 'r' from fouR in 4.

Great. And then mnemonic logic goes out the door. 5: 'l'. Ah. A single bar. Err.. is it 1 or 5..? 7: 'k'. Imagine the K looks like a 7. Ok, maybe.

So the mapping itself starts getting confusing. Unless you do what they claimed all along the book you shouldn't do: rote memorization.. Which was grinding my gears a bit..