On September 12th 2018, Apple announced the upcoming release of iPhone XS and iPhone XR series smartphones with Super Retina display, as well as the new Apple Watch Series 4. iPhone XS and Apple Watch series 4 will be available on September 21 and iPhone XR will be in stores on October 26th.
The Apple Watch Series 4 Health app can monitor your heart rate, and also includes Fall Detection and Emergency SOS capability. An ECG app that has output similar to a single-lead electrocardiogram will be available at a later date. Update: the ECG app and irregular heart rhythm notification is available as of December 6th 2018.
iOS 12 for iPhone and iPad will be available for download by September 17th. The iOS 12 new features list includes a Measure app that can automatically detect dimensions of rectangular objects. Apple Books is the replacement for iBooks and allows you to read downloaded books and listen to audiobooks.
Devices that support iOS 11 generally can be updated to iOS 12. An iOS device should be backed up before installing this update. iOS can then be installed by selecting Settings > General > Software Update.