If you don't know REH's works beyond Conan, he also wrote quite a few westerns, supernatural stories, detective stories, pirate yarns, and so on. (He wrote a bunch of boxing stories, but I haven't read most of those as the subject doesn't grab my interest. They may be great stories, however.) His iconic characters include Francis X Gordon (adventures in Afghanastan during the 1920's) and Solomon Kane (a dark paladin adventuring in Africa) as well as many others. He wrote about all sorts of eras of fiction, from Kull (age of Atlantis), through Conan (20,000 years ago), Bran Mak Morn (Roman era), Black Vumela (pirates of 1600's) and Solomin Kane, to the "modern" era detectives and Cthulhu mythos style horror. (Keeping in mind that "modern" was 1930's, the time when he died.)
As best as I can tell, all of Howard's characters were supposed to be co-existing in the same historical timeline of our Earth. I think it would be fun to have AA material specific to a "REH Earth" campaign, where one could battle Cthulhu-style evil while searching for historical artifacts, or even one where time travel was possible and one could adventure back to encounter Kull in Atlantis or some other figure in REH's timeline.
If only someone would write this book!
If you want some playtesters, I think my group can find time in our schedule....