In chapter 35, the reader is asked to run a command which is not later discussed

[Jason A. Ribeiro]

In chapter 35, the reader is asked to run this command:

:echo join(foo)
:echo join(foo, '---')
:echo join([1, 2, 3], '')

The book goes on to say that:

Vim displays "a b" and "a---b". join will join the items in the given list together into a string, separated by the given separator string (or a space if none is given), coercing each item to a string if necessary/possible.

but does not mention "123" (the output of the third command).

[Steve Losh]

This got fixed at some point.