Hey there. 400 pages does seem like a lot, but the vast majority of the book isn't stuff you need to get up to speed with. There's a chapter that walks you through settting up your character and setting, a ton of random tables, and bunches of content that's really only needed when you have a question or want clarification about some aspect of the game.
I'd suggest you to take a look at the first chapter, which is a free download on my website (https://tomkinpress.com/collections/free-downloads/products/ironsworn-starforged...). This will give you a good sense of the game, along with maybe 95% of what you would need to know to get started playing.