Sameer Bhangar the Test Lead on ClickOnce project recently internally sent out a list of the top ways to learn about ClickOnce.
Is by no way the authoratative or complete list but definately good starting point. This should serve as a good resource for anyone ramping up on ClickOnce.
Feel free to recommend other resources and I'll add them in here.

Top X ways to learn about ClickOnce -

ClickOnce Discussion Forums:
    External forum - 
    - As I as have mentioned earlier in my blogs, the forum is the ideal site to go to with ClickOnce issues, bugs, feature request and even general discussion on App Model/Deployment. It is looked at regularly by the ClickOnce product team who do a great of following up on the posts.

Microsoft sites:
    Smart Client Dev Center Home:
    MSDN content on ClickOnce -,vs.80).aspx.
    Video of Jamie Cool the PM on the Winforms/ClickOnce team demoing ClickOnce -
    Article by Brian Noyes on configuring trust with ClickOnce -
    Article on ClickOnce and Regfree COM -

    Overview article by Brian Noyes -
     - Bit old so some specifics may have changed but overall is accurate    

Non-Microsoft sites:
   ClickOnce Publisher Certificates - 
   Links to various ClickOnce related material -
   VS Support for ClickOnce -
   Another VS walkthrough -

 Few Blog references to ClickOnce:
    Saurabh's blog -
    - The one stop Authoratative ClickOnce Resource (well Sammer's mail didn't say so but it should have :-) )