"Premature optimization is the root of all evil" (D.E. Knuth)
Efficient code can be well-structured and clean, based on a sound overall architecture and sound algorithms. Efficient code can be highly implementation-code that bypasses documented interfaces and takes advantage of obscure quirks in the current implementation.
Ideally, your code only contains the first type of efficient code. If that turns out to be too slow, profile the application to find out where the performance bottlenecks are and optimize only the bottlenecks. Let other code stay as clean as possible.
This Efficiency Guide cannot really teach you how to write efficient code. It can give you a few pointers about what to avoid and what to use, and some understanding of how certain language features are implemented. This guide does not include general tips about optimization that works in any language, such as moving common calculations out of loops.
It is assumed that you are familiar with the Erlang programming language and the OTP concepts.