Secret Bookcase Door Diy . Secret door bookcase step 1: First, measure and record the height, width, and depth of the entrance to your secret room. DIY Hidden Bookcase Door Your ProjectsOBN Bookcase door, Bookcase from www.pinterest.com Calculate the dimensions of your space. The total depth of the hidden bookcase door should not exceed 8, this avoids the chance that the case hits the. Measuring and marking 40 inches (see note below) back from line #2, another perpendicular line (line #3) is drawn parallel to line #2 and will be.