Question: I am currently learning OLAP & MDX after many years of relational database development. 

Any tips on getting started in MDX? What are the best books and resources to learn MDX?

Answer: 
A classic, albeit a bit dated, book is [Fast Track to MDX](https://rads.stackoverflow.com/amzn/click/com/1846281741). It's a great overview and a quick read, though it doesn't cover the new MDX features of SQL Server 2005.

The Spofford book [MDX Solutions](https://rads.stackoverflow.com/amzn/click/com/0471748080) is more up date and a little deeper, but a bit harder to get through.

I also highly recommend the blogs of [Mosha Pasumansky](https://web.archive.org/web/20180325203953/http://sqlblog.com:80/blogs/mosha/default.aspx), [Chris Webb](https://blog.crossjoin.co.uk/), and [Darren Gosbell](https://web.archive.org/web/20210211164051/http://www.geekswithblogs.net/darrengosbell/Default.aspx).
Here is an [MDX gentle introduction](http://www.iccube.com/support/documentation/mdx_tutorial/gentle_introduction.html).