If you have an iphone, you should automatically have the iBooks app. Search up the book called “Music Theory for Guitar”, the one with the acoustic guitar book cover/thumbnail is the one I’m talking about. It should be free (it was for me atleast) and alot of the information is easy to understand (along with video samples) There’s 87 pages in total.
If you’re looking for a more general music theory source of info, I’d recommend musictheory.net. Its a very organized website with great visual explanations.
Good-day.