You need to get read up on the theory behind data warehousing first before tackling SSAS.
I recommend Kimball's Data Warehouse Toolkit Classics, then if you want to follow it up with the Microsoft Data Warehouse Toolkit it will give you some good examples to go off of where the theory is applied.
After that you probably want to get up to speed on MDX. I recommend the books Fast Track to MDX even though it's based on 2000 technology, and though I haven't read it, I've heard that Microsoft SQL Server 2008 MDX Step by Step is a good book too, although unneeded if you already have some MDX under your belt.
You can then go with whatever SSAS book you want to learn more about the tool itself if you feel like you need to, I haven't read any of these so I'm not comfortable recommending any.
You should be able to find all of these on Amazon by searching their titles.