"Why the Sky is Blue"

Solution 1:

"Why is the sky blue?" has the grammatical structure of a question, and cannot be interpreted in any other way.

"Why the sky is blue" has the grammatical structure of a phrase standing in for a noun; it could be replaced by "the reason for the sky's blueness" or "the reason the sky is blue". E.g. one can say "Why the sky is blue is a fascinating question". It can also be interpreted as a question, but technically this is not grammatically correct.

Solution 2:

It's very simple, you are adding the question mark. That is why is reads funny to you and the other posters.

The book title is NOT a question. It is a statement.

The presumed question is, "Why is the sky blue?" Because David Richerby understands how Rayleigh scattering works, he could write a book explaining it to you. He might call that book, "Why the sky is blue."

A similar phrase is used in "I know why the caged bird sings" by Maya Angelou. It could have been called "Why the caged bird sings."

Solution 3:

It's all about 'subject-verb inversion' which is something that happens in English when certain kinds of element occupy initial position in the clause:

[1] The sky is blue.

[2] Why is the sky blue?

[3] *Why the sky is blue?

Ex [1] is a simple declarative clause with "the sky" as subject and the auxiliary "is" as the verb. Inserting the question word "why" at the beginning triggers subject-auxiliary inversion to give the interrogative [2]. Likewise in [3] "why" has been inserted at the beginning, but this time no inversion has occurred so the clause becomes unacceptable in Standard English.