Vidas Matelis made up a more extensive reading list here. My top five reading list. Read these and also if you can bring them as reference material on eBook format.
The Maestro has no doubt changed a bit since I went in June 2011, but I would recommend making sure you are very familiar with all the references below:
1. SQL Server 2008 Analysis Services Unleashed
If you read only one book before doing the Maestro – this is it. Be warned it is a huge tomb of book. Myself and Michael Barrett (who also certified as a Maestro), both carried this around in eBook form on the iPad and constantly referred to it, especially WRT memory internals such as how the cleaner works. The exam was open book, so keep the books handy!
2. Analysis Services 2008 Performance Guide
You must read this until you actually get the concept of MaxThreads, CoordinatorExecutionMode and all the other joyful settings for Analysis Services.
3. Analysis Services 2008 R2 Operations Guide
Similar to the Performance Guide, a must read BEFORE attending the Maestro course.
4. Many to Many Revolution 2.0 and Optimisation Techniques
5. Expert Cube Development with Microsoft SQL Server 2008 Analysis Services
Chris Webb, Alberto Ferrari and Marco Russo is to Analysis Services what the Kimball Lifecycle books were to dimensional data warehousing: Something most consultants carry around in their bag when on a customer site.