This is a book I read in about 1975 at the age of 9. The book was targeted for that age or maybe a little older. I recall it being comparable to the 'Danny Dunn' books, but I don't believe it was part of any series.
The main character is a boy of 10 or so, and there is also a girl of about the same age, and possibly a dog. The boy's father is a scientist, and I think he goes missing, captured by aliens that are covertly investigating his research. The boy and friends eventually save the day, of course, though I don't remember how.
The book was 8.5" x 11" sized, hardcover, and I believe the cover was purple, very purple. There may or may not have been other stories in it.