It seems that the folks at Wildside Press have obtained the rights to the Danny Dunn stories! These have been out of print for far too long!
The first two titles, "Danny Dunn and the Anti-Gravity Paint" and "Danny Dunn on a Desert Island," are currently available. $3.99 each, and DRM free, I believe. Looks to be the original cover art, too!
http://www.wildsidepress.com/search....dunn&submit=Go
A sampler with the first few pages of each book is available for free (perhaps only for a limited time):
http://www.wildsidepress.com/freebie.htm
Now if someone could only locate Alfred Hitchcock's Three Investigators, I'd be all set.