Writers do half of the work of creating a book. Readers do the other half.

Finding meaning in the book is the reader's job as much as it is the writer's.

If a teacher assigns you to read a book, and asks you to analyze its layers of meaning, there's a good chance your teacher has chosen a book that was written with layers of meaning. Yes, the writer probably did intend for it to be read closely and thoughtfully, and probably wanted you to find layers of meaning in it.

If you apply the same methods to a book that was written purely to provide a few hours of entertainment, maybe it wasn't written intending that you read it closely and thoughtfully, finding wisdom in its layered meanings, but that doesn't mean that, as a reader, you can't find wisdom there anyway.

TLDR: Yes, Steinbeck really did intend something deeper with "Of Mice and Men." No, John Grisham probably didn't intend anything terribly deep with "Rogue Lawyer." But that doesn't mean you can't read "Rogue Lawyer" deeply if you want to, and see what you find there.