THE MAC VALVE STORY 28-page booklet Mechanical Air Controls (1957) MY GRADING SYSTEM I try to be fair and honest in my grading, and make every attempt to mention loose pages, stains, binding damage, etc. Please understand that the glue used in the binding of old paperbacks was not designed for longevity; when old paperbacks are opened for reading, additional pages may loosen at that time. EXCELLENT: in exceptional condition; not perfect, but very high grade. VERY GOOD+: still nice, but showing its age; some overall wear. VERY GOOD: the average used paperback; not heavily read, but shows an accumulation of wear and creases. GOOD: a complete copy, but well-read with worn edges and a greater accumulation of creases and other wear.