The MongoDB user group (https://groups.google.com/forum/#!forum/mongodb-user) has a great archive of user questions about features, and long-standing bugs. It's a place to go when something doesn't work as expected.

Online forums (Stack Overflow, reddit, among others) are always a source of knowledge with the trap that something may have been posted a few years ago and may not apply anymore. Always check before trying.

And finally, MongoDB university is a great place to keep your skills up to date and learn about the latest features and additions: https://university.mongodb.com/.