How do I compare different authors to each if their works are about different things?

Often, it is more about how an author says something rather than what they say. So, the books you're reading might be about completely different things, and have different subject matters, but the way that they express their works might be similar.

For instance, their writing might use a lot of different literary techniques (eg, metaphor, simile, descriptive language) so you need to look at the way they use language and see if there are any similarities there. You also need to look at the different themes they explore. Sometimes a scene in a novel may be simply a literary device (something, perhaps a character, or an event that authors use in order to make a point, or express a point of view) in order to show something else, so look at the events and characters in the novels and see if you can think of any deeper meaning or significance behind them, and then compare them. 

Sometimes, it is the difference between authors that is interesting, so if there is an aspect that is completely different, it is worth mentioning. This difference may actually highlight something important for the individual authors!