The method is correct. Just you have to know what is behind.
-> 93 x 94
100 - 93 = 7
100 - 94 = 6
-> 93 x 94 = (93-6)x100 + 7x6 = 8700 + 42 = 8742
-> 7 x 6
-> or other way around: (94-7)x100 + 7x6 = 8700 + 42 = 8742
Important is you take the 100 as a base for the calculation. For 32 x 35 you can take the base 40. In general it works with "any" base.
For the book, have a look at the amazon and search for "mental math".