Its approximately 70 pages long. I found this invaluable in learning how Magento 1 worked. Put simply it was a short guide that produced an extension from start to finish teaching many basic principals of Magento 1 such as:
Code setup and configuration - code structure
Design setup - templates and layouts
Frontend development - database schema, models, output (blocks), routes, helpers, sample data
I believe where this guide succeeds is that it carries a single example across many topics. More importantly it has the right level of detail in terms of producing something useful yet not too much to becoming obsolete overnight. And it focuses on magento in terms of PHP development unlike some of the books I've read through which cover a far wider range of topics and often include a few chapters on installation and setup for example.
I realise that Magento 2 is technologically more advanced and perhaps this is too difficult to achieve. However reading through the developer docs many areas still seem disjointed (and by this i mean a common example which is carried throughout - not simply a collection of snippets of information) and I feel it would be really helpful if an all-in-one guide extension guide was put together not only for me but to help the greater community.
To give you a bit of history I'm a competent Magento 1 developer but I'm finding some Magento 2 concepts difficult to follow.