How do I find an iPhone 15 user guide which was downloaded in the Books?

Hard to say, there a bunch of iPhone 15 guides in the Books App, but not many are free outside of Apple's official guides.

I would suggest you search for iPhone 15 guide and see if any match what your looking for or have a cloud icon indicating you downloaded them before.

One I see that is free and not from Apple is: https://books.apple.com/us/book/ios-18-user-guide-handbook/id6738393448

cjbruce2015 wrote:

When I was in my local Apple Store recently, one of the staff highly recommended a user guide to help me with my iPhone 15. He downloaded it for me to Books. Now I look on Books and it looks like it didn't save. The trouble is I can't remember the guide's name. It wasn't something like iPhone User Guide. It had a more nuanced, hard to remember title. Does this sound vaguely familiar to anyone on this site?

click here https://support.apple.com/en-us/docs/iphone/300991