As I started in 2005 with DotNetNuke it looks to me as a very secure, stable, and well supported CMS.  With the work of DotNetNuke over the years I learned many stuff, and found out it is easy to develope modules on it.  As a .Net application you could develope your modules on c#, vb, or any other from .Net supported language.

I am not a developer but I was able to develope my own application using the wiki, and other articles about DNN module developing.  So I don't think it is havy to learn how you could develope your own modules. 

Also the administration of DotNetNuke looks to me very easy to learn.  It have a very good user expience and is easy to understand. 

The DNN 6.2.1 have social functions integrated and a blog module is availible too (new version comming with social integration soon).  I am not sure if the Wiki module that is availible for free is what you want to use, but I am sure there also solutions from 3rd party vendors.

Personaly I mean DNN is a very simple and good solution for your CMS.  Basicly you still can use books from older versions to learn, the most basics are still actuell with some enhancements you could found on blog posts, wiki and articles on the www!

With well supported I mean that you got a lot of help of the community on the Forums, you got resources on the Wiki page, starting instructions on podcasts (video), Blogs and 3rd party articles on the www. 

Of cause books are not on the same version, about it need a lot of time the authors need to write new versions and new versions comming out faster.   But from version to version there are only enhancments and fixes, so books from version 5.0 could still be used. Enhancements are announced on the blog and documented on the wiki, so I think you could found anything you need.

For starting with DotNetNuke I would suggest all the videos you could found on this page and use the wiki! 

If you want a book I could suggest the following books:

Professional DotNetNuke Module Programming (Wrox)  by Michel Sellers

There also some books about skinning (designing) and administrating. 

If you want more information about developing on DotNetNuke, I think you should start with the Visual Studio Starter Kit package.  Try it and found out how easy it is to create your own applications as a module.  Of cause you could use DotNetNuke also as a Frameworks and use its code as a reference in your projects.

If you want to know how using DotNetnuke and existing modules (free modules could be found on codeplex.com) play with a instance on a local system.  It is very easy to use!

I am not a developer, I am an admin, but it was easy for me to create my own first module and create own skins.  So I think as adeveloper you have more understanding it easy how you can develope your own module.  Please check the resources on this site.