I'd add Apple's Human Interface Guideline document and annual WWDC sessions on user interfaces. Even if you don't use a Mac or iOS device, or even if you hate Apple, taking the time to understand why they think their user interfaces are the best on the planet goes a long way to being able to understand why other companies also think their user interfaces are the best on the planet. The ideal result isn't a happy medium, though; it's whatever suits each platform best, something you only understand when you know what each platform's designers are going for.

Don't look into Microsoft's design documentation though; even they can't decide what their design principles are. The only time they did, and stuck with it to the end, was Windows Phone.