I've mentioned before my nostalgic love of Tom Swift Jr. Books by Victor Appleton (pseudonym). Most of the original Tom Swift books by Victor Appleton are available for free download on a number of sites, but one can find relatively few of these more recent books. Tom Swift and the Electronic Hydrolung is available for free download from Manybooks.net and other free book sites. I still remember fondly reading it for the first time. It must have been about 1972, though the book was published in 1961.