In the adventure genre there's Tom Swift, The Rover Boys, and other series mainly from the Stratemeyer Syndicate. A good list of books available online can be found at
http://oldfashionededucation.com/fictionforboys.htm I will point out that some of these series feature older teens and in some cases, boys joining the military and going to war. Those are probably just a bit too old for an 8-year-old.
Boys often enjoy historical fiction. You can find many of the books written by G A Henty here at Mobileread as well as those by Howard Pyle. Stories about Ancient Greece and Rome may be a good choice. You can get The Age of Fable (aka Bullfinch's Mythology) in the MobileRead library as well. And don't overlook biographies. A list of biographies suitable for kids can be found at
http://oldfashionededucation.com/biography.htm
Someone has already mentioned The Bobbsey Twins. In that same vein, look for The Five Little Peppers and Elizabeth Nesbit's series that begins with Five Children and It. I've not seen them in electronic editions, but I'm really fond of Enid Blyton's Famous Five series and Arthur Ransom's Swallows and Amazons series.
And, most kids enjoy fairy tales. All of the "color" books by Andrew Lang are here at MobileRead -- and of course, Grimm's Fairy Tales.