Dorothy L Sayers, from a short essay entitled A Note on Creative Reading (from the back of a book called Begin Here, which was published during the Second World War and deals with economics and Germany and politics and a good deal more).
Which reminds me: please burn all your book-markers - even the pretty one Aunt Mabel sent you last Christmas (or at least put that one away and only bring it out when she comes to call.) You cannot possibly be so bird-witted as to be unable to discover which page you got to by looking at it.
If the author mentions some other book in terms which make it seem important, whether he approves or refutes it, don't take his word for it: get the other book and read it, and judge for yourself. If he refers to something, or uses a word, which you don't understand, get a dictionary or work of reference and look it up. (Don't write and ask the author to explain; he is not required to be an Encyclopaedia, and you will only give him a poor idea of your industry and intelligence.) Especially, examine the sources of what he writes; to read Mr Somebody's critical valuation of Milton's prose or his examination of the economic effects of the Peace-Treaty is quite valueless if you have never read any Milton and do not know what the Peace-Treaty actually said.
How times have changed, you might say. I don't at all agree with her view of bookmarks because I'm plainly bird-witted enough to need them to find my place. It must speak volumes about my lack of attention to what I'm reading (!)
As for asking the author to explain: these days authors are all too ready to explain, and enjoy discussions with their readers. Or so we're led to believe. It's one of the joys and perils of the Internet. You can drop an author an email or even a tweet without blinking. And mostly they will reply...quickly.