As a learner of English, how many is the number of books which is appropriate to work on reading at the same time?

If the number is too many, it would become more difficult to keep track of the content and the story of each book. But if you're working on reading only one but difficult book, much of your mental energy must be invested in trying hard to understand the content itself, making it hard to parse as many sentences as you can.

Having at least one easy-reading book at my hand would be desirable.

Going on reading a book and parsing and examining sentences one after another feels a bit like a fieldwork of a zoologist. I encounter peculiar sentences, eloquent sentences, restrained but clear sentences, very long sentences, etc., one after another...