JRR Tolkien is the best perspective shift writer I've ever read.
When I was in college, I took a writing class. In that class one of the very first things that they taught was to never, ever, under any circumstances, switch the perspective of your writing. Now, there was a reason for why they forced us into this, because they wanted us to stick to either first person or third person for our writing, and they believed that when you switched perspectives too much is messes with your ability to keep that straight, and it messes with the readers ability to follow what you write. But there was another secondary reason that my teacher gave after the lecture. She said, essentially, the reason I want you to not switch perspectives when you write a story is because it’s really hard to do that in any sort of convincing way to your audience. It’s just flat out hard to do it well. Plenty of authors do it, but we needed to learn the rules before we learned how to break them.