I am currently reading a book on a research-level science topic. But there are some concepts that aren't properly explained in the book and are stopping me from proceeding. I have tried the following:
Searching the Internet and the library for these concepts
Asking about these problems online
But these attempts haven't been very useful. And I am quite desperate now. I am now considering writing an email to the author asking for clarification directly. Yet I don't know if this is an inappropriate or rude thing do? And if I do do that, how to avoid being impolite?