By Louis Slobodkin.
This is part of the "Eddie and Marty" series (I guess those must be the names of the kids in the book, though I have no memory of them at all). The books in the series are: Spaceship Under the Apple Tree, Spaceship Returns to the Apple Tree, Three-Seated Spaceship, Round-Trip Spaceship, Spaceship in the Park. I only ever saw the first one.
Aliens in the back yard. Very fond memories. I never knew there was a series. I think a lot of SF fans start out really liking libraries; they are, after all, wonderful places full of wonderful books we haven't read before. Good libraries even had those little rocketship stickers on the spines to identify the good books (or were they atoms in your library?). I may have read this book out of my third grade library (which was in the school across town from home). I have vague visual cues that suggest that library to me, but I don't know why.
Anyway, as happens to many SF readers, they change schools, or change towns, or something, and they find that the new library has lots of wonderful books in it -- but it doesn't have some of their old favorites. I think lots of us got used to rereading old favorites because there simply wasn't that much SF around (and because rereading is a common reaction to really good books), and being cut off from the old favorites is very hard. In my case it was changing schools.
Suddenly libraries seem more of a mixed blessing. It's still wonderful that there are these places filled with books we can read, but suddenly we've learned that they can be taken away from us unexpectedly. And that, I believe, is why so many SF fans own so many books.