This website is a great resource for the DAX formula language used in PowerPivots in Excel 2010 and Excel 2013. The site contains solutions to common reporting requirements in PowerPivot Reports They have also published a book based on the content which is cheap as a Kindle edition, I have recently bought it.
SQLBI – this website is a great resource for the PowerPivots in Excel 2010 and Excel 2013. Some of it is quite Advanced. See also the DAXPatterns site. These guys have published a few books and I own a couple of them.
27 Excel experts discuss Excel and its future with Business Intelligence.
Some great insights – worth a read.