Well... clearly, the most underrated is likely the least rated book, no? So for that reason I will suggest,
Electronics Assembly Methods, Duarte and Duarte. It's old (60s era), hardly useful or pertinent, and the only reason it comes to mind is because it was a random book that I looked at a lot, growing up.
So there you go, I challenge you to find a review of it online and disprove my dubious interpretation of your question.
Speaking of "underrated" things, there's the Mark Twain(?) quote,
"One of the most overrated pleasures in life is intercourse, and one of the most underrated is defecation."
Tim