As I’ve noted before, the Discord channel for your class is a great place to get information about how to play your class and spec well. I recently checked the https://www.reddit.com/r/competitivewow Discord channel list, and wanted to post a list of class channels here as well for your reference. If you want to make sure that guide you read in prepatch is still up-to-date and search for more resources, the Discord channel for your spec is a great first stop. I will be going through and cultivating the class guides, hopefully in time for the release of mythic raiding. In the meantime, your class Discord is a good first place to check for up-to-date information, and can be all you need in some cases.
For even more Discord channels, check out this list on Reddit: https://www.reddit.com/r/wow/comments/4yfgms/wowclass_discord_info/.