Teach Yourself iOS 8 in 24 Hours was written with the latest version of Xcode, and the project files were prepared with the latest beta. Unfortunately, Swift is evolving quickly. There will likely be small issues that occur in the project code as underlying changes are made. In the GPS projects, for example, you’ll see an change to the downcasting in Xcode 6.3 that requires “<blah> as! <blah>” as opposed to “<blah> as <blah>.” Xcode 6.2 (the current shipping version at the time of this writing) doesn’t require this.
If you have a difficult time with a piece of code in the book, let me know! I’ll make sure that an update is made available.