It might be hard to read because of the image resolution, but on the cover of this book, on the bottom left side, it says:
Which means something like:
"Even strange Japanese has a reason."
The thing is, 理由 has furigana above it that says it should be read わけ.
わけ is not a reading for 理由. 理由 is only read りゆう, and it means "reason".
Now, I already know that sometimes furigana is used to differentiate between what is said and what is meant. For example, recently in a manga I saw the kanji 妻 with the furigana ワイフ above it. I believe this is done when the author is telling us that the character said "wife", but the kanji 妻 is provided so if the reader doesn't know what the katakana-ized English word means, they have the kanji to help.
So, in a way, when the furigana and the kanji differ, it's so that the kanji provides meaning to the furigana, as opposed to furigana's usual job of providing readability to the kanji.
Okay, fine... but, why わけ for 理由? Doesn't わけ also mean "meaning", "cause", and even "conclusion based on reasoning"?
The difference seems so subtle to me that I can't see the point of it. Yet, given that the book is all about Japanese usage, I imagine the author is quite particular about words, so surely there's a purpose.
What is that purpose?
(Side note: I intend to buy the book, so maybe an explanation is within its covers. But I hope this still merits some discussion.)