If you are into podcasts then I recommend The Semi Pro Cyclist (semiprocycling.com), it's definitely leaning more towards road bikes but he covers mostly general concepts that have a lot of carry over. The host is clearly passionate about cycling and training and is not afraid to get nerdy about it, so you may need a strong geeky side to get into it. What I appreciate most is that he puts effort into researching the topics and more often than not will references multiple legit studies from various viewpoints, and the fact that's he's not dogmatic about the "best" way to train is a welcome change from a lot of what is out there online.
On the other end of the spectrum is bikejames.com, he's about as opinionated as they come and a bit of a cherry picker about science that backs up his claims, but I happen to agree with is views on how beneficial strength training can be and other ways to become a better mountain biker, such as riding with flat pedals often.