спасибо всем

German, I don't know what they're trying to teach you, but we don't say "Books written by whom do you prefer?" and we don't say "Book by which author do you prefer?" These both sound highly unnatural. You won't find one example of either of these on Google or in books. We simply say "Who's your favorite author?" or in rare cases "What author's books do you prefer?"

Alex, your sentence is correct grammatically (and contains BY WHOM as requested), but nobody would actually say that in living English.

Valery, your sentence, although still highly unnatural, would sound much better with WHAT author.

When I was typing that text, a couple of teardrops heavily fell down on my sofa😢

With good reason!

Александр, Вы знаете, что надо сделать. К Валерию обращаться бесполезно. Похоже, он робот.

He never responds

Ага😏