$1 is my favourite resource for learning and teaching voiceover. If your mother can use a computer, she should read the lessons on her computer, and try the tutorials on her phone. If not, you might need to go through them with her. Reading the lessons on your phone, while learning, really won't work.

As for apps, VoiceDream Reader is probably the app I spend the most time using. It makes it as easy as possible to get books on my phone, from multiple sources. If your mother isn't part of Bookshare, she should be; it's totally worth it. And VoiceDream Reader can download and read books directly from bookshare, without needing a computer or anything.

I use Google Music for my music streaming needs, because it's the most accessible of the available options. Anyone who has completed the lessons from Voiceover Easy won't have any trouble with it. Interestingly, Apple Music still has bugs with voiceover (you can't scroll past track 20 on any album with more than 20 tracks, for example).

I also use TuneIn radio for all of my sports and talk needs. The app is perfectly accessible.

Blindsquare is my GPS app of choice; nothing beats it.

And those would be the apps I use daily. I've got lots of others, but I might only open them once a week or less.

BlindSquare, a very detailed GPS app.

For getting better at VoiceOver, it is mostly practice. But there are also videos and other resources on-line to learn different things as well.