Early readers are a whole world, and the transition from picture books to chapters is huge. My kid, who's six, went through this last year. The sheer excitement on his face when he finished a whole book by himself was worth all the trial and error. We tried a bunch, but the real winners were the ones that didn't feel like a chore.
Series like 'Henry and Mudge' were a godsend. The sentences are simple, the chapters are super short, and the stories about a boy and his big dog are just... warm. He could read one before bed without getting frustrated. 'Frog and Toad' is another classic for a reason—the friendship is gentle and the humor is quiet, perfect for that age. I'd avoid anything that looks too dense on the page; white space is your friend. 'Mercy Watson' by Kate DiCamillo is also brilliant. It's sillier, with a pig who loves buttered toast, and the illustrations are integrated so well they act as supports for the text.
For the more adventurous, 'Narwhal and Jelly' is a comic-hybrid that flies off the shelf. The graphic novel format tricks them into reading more than they think they can. The key is matching the book to the child's mood, not just their reading level. My son will still pick up his worn-out 'Henry and Mudge' book sometimes, and I think it's because it feels like a friend, not homework.