Sweet Valley High series
Ahhh - happy memories
I also loved Nancy drew, as well as Mallory towers and st Claire's, and the complete little women series, where in fact my fave was "Joe's boys". I progressed to georgette hayer, and then by the time I was 12/13 got seduced by my big sisters slightly more raunchy selection.
The boys are all completely different, they only share a dislike for Harry potter and Lord of the rings. Ds1 loves adventure books, thrillers and humour. Ds2 likes quite serious, moving books, particularly based around WW2 Michael morpurgo war stories, boy in striped pj's and currently the really superb "run rabbit tun", he also loves Percy Jackson.
Ds 3. Loves reading, far more than the other two, he really enjoyed the shorter gothic illustrated series by Pullman..particularly the scarecrow and his servant. He is currently reading "I am David", but he also adores short Greek mythology stories and is not adverse to a bit of "middle school" to read in bed, but reads them too quickly to justify their cost, and the library has not hot them on the shelves.