I think Harry Potter is the best chapter book series that you should start with. While the books are big, a 8 year old child could probably finish one in several weeks, assuming they read for about 30 mins every day. There are 7 books in the series, each one building off of the last with its own unique plot, keeping the series fresh throughout. After you read Harry Potter, I would recommend the Ranger's Apprentice series, which is also an amazing series that teaches you about loyalty and bravery.
In this series, Harry Potter, who Voldemort attempted to kill, joins Hogwarts, a school for magical students, and meets Ron and Hermoine, who end as his best friends. Voldemort has many people with him, and together they get Harry into quite a bit of trouble, but often, the trio get themselves into trouble. I would say that the significance of this series is that it shows you with love and friendship, good can always win against evil, no matter how strong it is.
I read each of the books many, many times when I was young, and I really loved reading them. There are also movies for each of the books, and I really enjoyed those, because unlike a lot of other movies, they didn’t differ much from the books, and it made the movies much more enjoyable. I would recommend this book for ages 6+ because of how J. K. Rowling managed to make these books elaborate, with a lot of detail, imagery, and description, but at the same time they weren’t overwhelming. This all contributes to this being my recommended first chapter book series to read, with a 4.5 Star rating.