This post from SqlBatman describes a situation similar to my current client and, in reality, indicative of many companies that rely on tons of reports which have been shifted to over-night processing because of their performance impact.
How do I get started using Analysis Services in general and OLAP Cubes in particular to help my clients?
I really like Analysis Services Step by Step to get up to speed on all that SSAS can do.
In short, SSAS gives you quick access to pre-aggregated data. The real power of OLAP is not in being able to generate reports quickly, though; it's in being able to interact with the data. Slicing, dicing, drilling down, up, and through. It's a dataphile's dream.
That being said, if you just want to get static reports out of this, or you want to build out cubes, you really, really, really need to get up to speed on data warehousing. Grab a copy of The Data Warehouse Lifecycle Toolkit for this. Kimball's the authority on this, especially if you'd like to use SSAS on top of it.
The warehouse is the cake, the cube is the icing. Bake the cake, first, and the cube just gets put on top to make it that much better.
Cheers,
Eric