If all your blocks are 12" instead of 12.5"; it doesn't matter. Consistency is what you need so if all your seams are a hair more than 1/4"--it works out fine. If the problem is in your cutting--again--as long as you cut them all a smidge too small--it's ok. If the problem is in your pressing--that needs to be fixed.
As for good books--I agree about Maggie Malone's 5500 Quilt Block Designs being one of the first I purchased.
Paper-piecing might be the method for you. You don't worry about scants and 7/8's. You just sew on the line. Wrapping your brain around the flipping the paper over and sewing on the "wrong" side takes a little getting used to but once you get it; you'll have points so sharp they'll scratch glass!