怪人二十面相

If you like Nancy Drew or The Hardy Boys you'll like this book. The language is a bit old fashioned at times, but I think that provides even more learning potential as it's important to get exposure to that as well.

As a note, my version was a paperback copy containing footnotes explaining historical terms. I second the later review in that this will likely be easier read as an ebook if you're either unfamiliar with older vocabulary and/or you don't have access to an annotated copy.