Of course, the really impressive thing is writing books in parallel - form example Charles Dickens was writing Nicholas Nickleby while he was still engaged in finishing its predecessor, Oliver Twist.
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There are two types of parallel reading, it strikes me - one is where, for example, there's a book you read say when you are commuting, and a another book you read before going to sleep at night, and a book you dip into when you on the loo.

The other is the one I mentioned earlier, where you use an easy book to push and pull you through a more demanding one, so you've got to read the more demanding one to find out what happens next in the easier one. It also has the advantage of stretching out reading experience for the easier one.

There are a lot of books where there really can be quite an effort getting into them, and yet that effort is worth it, and once you are properly into them they take over. It's a bit like using tinder and twigs to start a log fire. The tinder and the twigs wouldn't keep you warm at night, but you can't just put a match to a log and set it blazing.

Of course, to extend that analogy, these days we are rather.. used to just turning on the gas...