DataAssist and eCart are developer tools aimed at making web development easier for a developer. These products cannot teach you web development, just like Dreamweaver (which is also not open source) cannot teach you web development.
What you are asking for is not a manual for using DataAssist or any of our other extensions, but a manual for dynamic web development (which DataAssist helps with just one aspect of). We have documented how to use DataAssist, but as a developer tool, there are prerequisites that you need to meet before you can jump into using it.
There are a lot of great resources on the web for getting started with dynamic web development. For beginners to get started with developing websites, we provide the PHP Database Starter Kit which should give you some insight into what it is going to take: database-for-php/