Thanks! There are no plans for an x86-64 version at the moment. The focus for me right now will be to incorporate all the great feedback we've gotten, fix the issues that have been reported and make it easier to contribute.
There is unfortunately no sample code for each chapter, but you can have a look at https://github.com/helino/aenix (the code in the book is based on aenix).
Me and Adam, https://github.com/tgwizard, wrote the book based on our experiences writing aenix, https://github.com/helino/aenix. If you find any issues with the text, please file an issue or open a PR at https://github.com/littleosbook/littleosbook
Please be aware that some typos and errors have been discovered, check the issues for more details!