If you have purchased the printed versions of either book in the US (and it was supplied by Amazon.com and not third parties) you can at any time add the Kindle version for only 99 Cents.
***Unfortunately this only applies in the US for the time-being!***
After reading the printed book, you can give it away as a present and still have the Kindle version for reference. Though the latter is not so good for perusing, it does have the advantage that you can enter a search term, such as a flight number or location, and go straight to it.