It is a difficult task to write a text book about just one Drupal module, especially when that module is not that complex, and a seasoned developer can figure out most of the options by reading the documentation or just starting to configure it.
The Drupal 7 Webform Cookbook by Vernon Denny (Kindle Location 301, Packt Publishing) does just this: a text book to cover the web-form module on 276 pages (print edition).
The cookbook describes the webform module in detail, while also covering related modules like the form validation modules, and an introduction to the Drupal hook system and the form api by giving easy to follow examples on how to extend the webform module and customize it. This makes it an excellent tutorial on how to understand a module and its components, and on how to generally extend Drupal modules.
I find the book a bit verbose and the text a bit too much of a narrative, would prefer more bullet points and tables to make it easier to just skim over the pages. However, that's a personal preference.
I find the Drupal 7 Webform cookbook very valuable as an introduction to - obviously - webforms, and more importantly to how much more you can get out of a contributed module when you are looking at all aspects, and use all tools available.
For full disclosure: I have been provided a free kindle copy of the book by the publisher for review purpose.