What is the value of 1939 Reader's Digest magazines of all 12 months?

It is difficult to say, because we don't know what condition your magazine is in, nor do we know how rare a July 1935 issue of Reader's Digest is. What determines a magazine's value is not always how old it is-- among the factors are how rare that magazine is, whether collectors are interested in it, and what condition it is in. Reader's Digest magazines were fairly common (a lot of them were in circulation), so they are generally not worth a lot, but there are always exceptions. You might want to go on a site like eBay and see how much similar magazines are selling for.