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 - http://forums.microsoft.com/msdn/ShowForum.aspx?ForumID=6 
    - 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: http://msdn.microsoft.com/smartclient/understanding/windowsforms/2.0/features/clickonce.aspx
    MSDN content on ClickOnce - http://msdn2.microsoft.com/en-us/library/wh45kb66(en-us,vs.80).aspx.
   
    Video of Jamie Cool the PM on the Winforms/ClickOnce team demoing ClickOnce - http://channel9.msdn.com/ShowPost.aspx?PostID=15303
 
    Article by Brian Noyes on configuring trust with ClickOnce - http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwinforms/html/clickoncetrustpub.asp
    Article on ClickOnce and Regfree COM - http://msdn.microsoft.com/msdnmag/issues/05/04/RegFreeCOM/default.aspx

    Overview article by Brian Noyes - http://msdn.microsoft.com/msdnmag/issues/04/05/ClickOnce/
     - Bit old so some specifics may have changed but overall is accurate    

Non-Microsoft sites:
   ClickOnce Publisher Certificates - http://geekswithblogs.net/kobush/archive/2005/05/30/41068.aspx 
   Links to various ClickOnce related material - http://www.installsite.org/pages/en/clickonce.htm.
   VS Support for ClickOnce - http://www.ondotnet.com/pub/a/dotnet/2004/10/11/clickonce.html
   Another VS walkthrough - http://www.15seconds.com/issue/041229.htm

 Few Blog references to ClickOnce:
    Saurabh's blog - http://blogs.msdn.com/saurabh/
    - The one stop Authoratative ClickOnce Resource (well Sammer's mail didn't say so but it should have :-) )
    http://mtaulty.com/blog/(5re21u3e1v3wjmm1qhgybqfo)/archive/2004/07/05/524.aspx
    http://mtaulty.com/blog/(32qtav45kx5a5zeb2taste3j)/archive/2004/07/06/525.aspx