The first book, "One Block Wonders" has general instructions on fabric selection and how to cut the repeating panels, then how to cut the 60 degree triangles for hexagonal OBWs, and how to cut 45 degree wedges AND squares for octagonal OBWs. Many of the illustrations for the hexagonal quilts show hollow cubes, but there are no instructions. The 3rd book, "One Block Wonders cubed" has the same info as the first book for fabric selection & layout, instructions for 60 degree triangle quilts and lots of info on hollow cubes, but no info on octagonal OBWs. BTW, the majority of OBWs seem to be the hexagonal versions (60 degree triangles)..... Personally, I would get the First book, then just go online to see how to make the hollow cubes. There are several sites, here's one:
http://articles.how-to-quilt.com/1112-hollow-cubes.php