A "reference album" is something that we use as a frame of reference when setting up a system, assessing a new component, or during the design. Thus, when we change something in our playback systems, by listening again and again to our reference music, it becomes easy to hear minute differences in sound that can sometimes be impossible to measure.
Beyond the reference, there is what I call the "absolute reference". This has to depict the actual sound, and I also have to be familiar with the real sound of the voice or instrument or performance in order for it to be a point of reference. This is most important during design as I don't want to be developing towards a "golden midrange" that doesn't actually sound real.
One of my favorite "absolute reference" albums is Blue Mind by Anne Bisson. Besides enjoying the music, it is an absolute reference as I know her voice live and un-mic'ed, and on LP, CD and high-rez download, it sounds spot-on. There are sonic cues in the human voice that make it instantly recognizable. Just as you can easily recognize the voice of a friend even in an extremely noisy environment, our ear/brain has evolved to instantly identify familiar voices.
Having Anne Bisson the singer, and Anne Bisson recorded makes it easy to design to what I call "Absolute Fidelity" for Genesis Advanced Technologies, Inc.