You don't need to buy them in order, no (although the way I got our set was a fabulously good value offer on the whole set, so watch out for one!) DS5 has only recently grown into them, so the only one I can comment on from his opinion of it is Numbers, the Key to the Universe, but that one is pretty good. I haven't read them enough myself to have an opinion.
I've also recently bought The Adventures of Penrose the Mathematical Cat, and The Number Devil. These both look pretty good to me, but DS has not read them yet.
