If you are just wanting to learn what umbraco is all about, I'd suggest you install 4.7 and learn the in's and out's of it; it is stable and feature rich, though uses webforms technology. There is a book on amazon that gives you a decent overview, and lots of help to be found on the forums at http://our.umbraco.org/forum
Once you understand what umbraco is all about, then take v5 for a whirl. Its not quite as feature rich yet, but having an understanding of all the features that were in 4.7 will give you a better idea what v5 will do when all the features and add-ons have been delivered. v5 is brand-new, just released and is a whole new architecture based on MVC.
For v5, install umbraco, and let it install the dev dataset. its a very simple application that showcases a few things about umbraco does...but there is definitely no 'dummy's guide to customizing umbraco v5' book out yet. It requires you to get your hands dirty and figure it out...or better yet, take one of the certification courses they offer and you will get upto speed even quicker.
http://umbraco.com/products/training