If you're planning to submit your apps to the App Store:
Starting March 2019, all new apps and app updates for iPhone, including universal apps, will need to be built with the iOS 12 SDK and support iPhone XS Max.
This will require Xcode 10, which currently supports macOS 10.13.6 High Sierra. But if the release schedule is the same as previous years, Xcode 10.3 in March 2019 will probably need macOS 10.14 Mojave, which only supports hardware from 2012 or later.
I'd suggest installing Xcode 10 now anyway. If you rename your installed Xcode 7.3, it can co-exist with the latest Xcode on the Mac App Store, which might have bug fixes for playgrounds. It will also allow you to try a newer version of Swift.
- Xcode 7.3 has Swift 2.2
- Xcode 8 (for your book) has Swift 3
- Xcode 10 has Swift 4.2 (with compatibility modes for Swift 3 and 4)
P.S. Please don't post multiple versions of the same question on these forums.