I recommand this book for SSAS/MDX beginners. Even if it is about SQL Server 2000, but the topics still up to date and applicable to the latest version of SQL Server. This book gives you a strong basis to discover and develop your MDX skills.
This is a great intro to MDX. I want to bang my head against the wall most of the time when I am trying to write in this language, and this helped to clear up some things with touples, etc....