Two that I would say are a must are:
- Billboard.biz : Billboard is THE music industry trade publication, and you need to read it. The subscription is a bit pricey, so if you can't afford it, reading Billboard.biz is a great option. Subscribing to Billboard Bulletin is also a fantastic idea, though it also requires a subscription fee.
- Hypebot : Hypebot is not only one of the leading Music Industry blogs, it's enjoyable to read.