Errata

The paragraph does not appear to have been updated from the original version of the Ruby cookbook. It states that the recipes were tested with Ruby 1.8.4 and Rails 1.1.2, which seems a bit unlikely in 2015!

The text that reads:
"In Ruby 2.1, a new syntax was introduced...."
Should probably read:
"In Ruby 1.9, a new syntax was introduced...."

I found at least the following mistakes:

The second paragraph text refers to a class called CardinalNumber whereas the code block below it has a class called OrdinalNumber.

The first line of code in the to_s method:
delegate_s = __getobj_ _.to_s
should be
delegate_s = __getobj__.to_s

The if condition uses a variable called "to_check" that does not exist. I believe it should be "check" that is given a value on the previous line.

There are checks for numbers eleven (11) and twelve (12) but, for example, the numbers 111 and 112 don't work as expected. They return 111st and 112nd when they should 111th and 112th, respectively.

The case-when structure is missing a when statement for number three to get the suffix "rd". It is given as an example in the comment block before the class implementation.

This is more like a minor technical inaccuracy and suggestion than a mistake per se.

Under the heading "Number conversion and coercion", it would be good to mention that methods to_i and to_f, and maybe others too, do not work well with non-numeric string input, i.e. on strings that don't represent numbers.

For example, 'abc'.to_i and 'abc'.to_f return zero and 0.0, respectively, which effectively hide an error that can cause hard-to-find bugs.

As easy as it is to use the method to_i, I think it should be mentioned that there is a better way to convert a string into an integer or float, for example, when accurate behavior is expected. The solution for that is to use Integer(str) or Float(str) that raise an exception if the string is not numeric.