

It's really up to you, what you're looking for in a book series, and where in an author's career you generally prefer to hop on the hype train.

Starting with Mistborn would be good because book 1 is written to where it could almost be a stand-alone (it was written before he had such a dedicated fanbase), it was written in a sweet spot where he was still early in his career and kinda rough yet far enough along that he'd found his footing so you can still get a good idea on whether or not you like his prose (one of the more consistent criticisms against him), and it's less of a commitment than SA. Starting with Elantris would be suggested purely because it's his debut novel and he only gets better from there (it's still a decent book, just not as good as it could've been) The other two popular books to start with are Mistborn and Elantris. SA is wonderful if you're into epic fantasy with grandiose scales, phenomenal worldbuilding, or simply need something hefty to show off to your book nerd buddies. That said, it's the one I started with and I've been hopelessly hooked ever since.

Due to how big each Stormlight book is and how much of the first entry is spent doing more worldbuilding and setup than anything, Sanderson himself describes SA as the series you read when you trust him as an author.
