There is quite a bit in the HTML of the box itself in terms of "wizards" and help pages and context help on most commands. I haven't seen a document with screen shots, etc.
You might just want to search for topics, such as DHCP, IPSec VPN, QoS or similar. Most articles are well-tagged around here and all of the most common ones include both GUI and CLI examples, along with sample use-cases and best practices. I think the GUI itself is pretty self-explanatory. Topical guides will aid understanding of features and functions. Of course, you're welcome to post questions as you go!
We do not have a manual specific to all of the features in Web GUI. The '?' mouse-over icons are often helpful, and as mentioned above, the feature specific guides often have web GUI sections. If you have a question about a certain feature or setting, feel free to add it here and we will be glad to help out.