Here's some great books to read:
This book is more like George Mueller's journal and everyday happenings. I am still reading this, it's takin me awhile.
And this is the other book.
This one is more like a biography and it starts from the beginning of his life, instead of a journal. I enjoyed this book a lot. It's easier to understand than his journal in case you want to read it. Might be best if you were a bit older to understand everything in his journal. He was a great man of prayer. He is one of my favorites.