is there a stereo blue book for value of old gear

Orion Blue Book issued each year. Used versions sometimes available here.

I believe it's called the Orion Audio Blue Book. Most Hi End audio shops should have a copy. You could probably buy one from a technical book shop or go to www.orionbluebook.com.

It really doesn't reflect true market values. It's primarily a tool for dealers.

Although I have not seen one in recent years, I always found Orion to under value equipment. My suspicion was that it's real purpose was to justify dealer's low ball quotes for trade ins.