Here is a link to my articles index, where I strive to make MDX, Analysis Services, Reporting Services and related topics understandable in a relevant (within the context of the business environment), interesting way.
Good stuff. There's a lot of useful information.
Nowadays there are a few good books about MDX e.g. MDX Step by Step, but if you want to learn quickly I recommend learning with videos, e.g. MDX tutorial