For me, reading while writing is like watching television: I can step into a story, get lost in it, while not thinking about what I’m working on, and then come out of it. It’s easy for me: a book ‘program’ can last for as long as you want it to. A book program can give you juice for your own writing (because seeing other writing to completion makes you want the same for your story), and a book program can make you think “my character wouldn’t do that because….”
There are non-program benefits to reading while writing too: you see how other writers pace their stories, how other writers describe their universe (much differently than yours – as it should be), you can see the way writers decide how much to share about their universe (a lot or a little), and you can see how characters change over the course a story (according to what’s happened to them).
The most interesting thing to me about writing is how you can tell a story about a world, or a character’s reaction to the world they exist in, and it can only be told for a certain period of time. You can only say so much: a day, a week, a month, or a year in the life of a town and its inhabitants or a character’s happenings is enough because a reader wants to know that they can think of what might happen to them after the story ends – even if you go far into their future in an epilogue.
Apparently, I think endlessly while writing and watching too.