The idea of reading more than one book at a time seems outrageous to some people. I can't even count the amount of times as a young tween that my mom questioned my ability to retain and keep two separate stories straight in my mind. In fact, she still questions my ability to do it. And although it doesn't seem like a special skill to me at all, and I didn't realize until later in life how much that skill translated into other things I do. Still, I had a lot of practice.
Reading more than one book at a time is a great brain training exercise to keep you sharp. Forcing yourself to learn the skills of separating details and keeping them individually focused in one's mind can help with organizational skills and general idea generating abilities. Reading more than one book and the act of switching between the two actively exercises your mind.
Switching from one mindset to the next and being adaptable is a skill much needed in general life. You may have a project at work that just isn't coming together and needs a new perspective. Maybe your friend is having a hard time and they've had enough of the stoic support and need tough love instead. This ability to be adaptable but still effective in any situation can be greatly tended to through the act of reading.
If it seems like a daunting task, try starting with two books you've read before. Reading novels where you are familiar with the plots, characters, and settings will make it easier to switch from one to the other and to set the first one aside in your mind when you do switch.
Trust me, it works. I know if you're not a reader forcing yourself to read TWO things is a lot, but it will be well worth your efforts.