To paraphrase Homer Simpson about beer, Roessler "is the cause and solution to all life's problems." It's the only Zenith reference available, so that makes it essential, but it has plenty of inaccuracies. Some have already come to light in this very thread, like Roessler's insistence in the face of common sense that the "Z" on the back of gold Captains has nothing at all to do with Zenith. I agree fully with Mike, the bigger problems are the yawning gaps in coverage. I'll also add the superficial treatment of a number of models, and the lack of judgement in selecting models to photograph. Frankens and obvious redials abound. Part of the fun of Zenith collecting is filling in the blanks left by Roessler.
Regarding the specific auction listing, Mondani is selling a large part of its catalog at deep discount, so feel free to make low offers on the listing