The question is, is it a book or a magazine? Pick it up, and it looks like a book. It feels like a book. But it's a "paperback magazine." The back cover, as you can see, tells us that it's "the very first paperback science fiction magazine."
How is it different from an anthology? Well, it has articles, but then so do anthologies. It has illustrations, but so could an anthology. I'd say that the distinguishing feature would be the columns. There's a science column by Jerry Pournelle, and there's a book review column by Spider Robinson. There's also the first part of a five-part article by Poul Anderson.
I think there were eleven issues (or volumes if you still think it's a book) of Destinies. Later on, Baen had his own publishing house and brought out New Destinies. I suspect that there are those among you who have collected all the issues (or volumes). I have most of them, but not all. They used to be fairly easy to find, and I suspect they still are.