The folks at Logos asked me to let you know about these resources. Its the Baker Academic Heidelberg Catechism Collection (2 vols.) It includes the introduction to the Heidelberg Catechism edited by Lyle Bierma, Charles D. Gunnoe Jr., Karin Y. Maag, and Paul W. Fields. This introduction is the state of the art.
The second volume is Caspar Olevianus’ Firm Foundation. It’s Lyle Bierma’s translation of Olevianus’ 1567 Vester Grund. The VG is an important representative of the sort of teaching that was being done shortly after the publication of the Heidelberg Catechism. There are verbal similarities with the catechism so this volume is an excellent companion to Ursinus’ commentary on the catechism. The hardcopy version of this volume has been out of print for several years so this is the only way to get it.
The set is available from Logos for $29.95.