The I believe that author gets the value of phi wrong: it is roughly 1.618, not 0.618.
The values are related by φ = 1/Φ, as well as Φ = φ + 1. Both of these relations can be derived from the fact that they are the ratios of a Golden Rectangle--if a square has length one, then adding a rectangle with height φ to the side will give a rectangle of length Φ, and the rectangles are Similar.
φ^2 - φ - 1 = 0
Which can be re-arranged, by dividing by φ to, φ - 1 - 1/φ = 0
or: 1/φ = φ - 1φ^-1=φ-1