Author Forums
Author forums are often the best place to find highly specific information. They’re also fun to browse. You’ll discover a lot about self-publishing, digital platforms, advertising and marketing just by aimlessly wading through posts and comments. You’ll also learn that most other indie authors are struggling with the same things you’re struggling with.
Kboards
KBoards includes a number of forums full of practical questions and answers. Topics include writing, marketing, advertising, and more.
Quora
Quora has dedicated topics for writing and self-publishing. There’s also a smaller section of self-published authors.
The subreddits self-publishing and self-publish are full of useful questions and answers.
KDP Community Forum
The KDP Community Forum is particularly helpful for Amazon, Kindle, and KDP-specific questions.