I also liked The Three Investigators and the John Bellairs books.
"Tikki tikki tembo-no sa rembo-chari bari ruchi-pip peri pembo" -- I remember that. Out comes the water, in goes the air.
Susan Cooper's "The Dark is Rising" books were incredible.
My favorite childhood book of all time:
Bear By Himself. Awesome, although my cover is better than that one. I know where my copy is at all times. It will go to my children.
Last, I also loved
The House at the End of the Lane but I haven't been able to find it. It has gone out of print.
EDIT: But I just bought a used copy off Amazon. Fingers crossed that it's as good as I remember.