‘Whiskey’ is one of those English words, like ‘aging,’ ‘center,’ ‘color,’ and ‘maneuver,’ that Americans and Brits spell differently. There are hundreds of them. In most cases, Canadians side with Great Britain, though sometimes they're with us, and sometimes they find a third way.
In the United States, ‘whiskey’ is the preferred spelling, as it is in Ireland. In Great Britain, Canada, Japan, and just about every place else, ‘whisky’ is preferred. With the exception of Canada and the handful of Americans below, everyone who spells it ‘whisky’ makes a barley-based, scotch-like product.
Several American whiskey brands, most prominently Maker’s Mark, George Dickel, Old Forester, and Early Times, use the ‘-y’ spelling. Maker’s Mark and Brown-Forman (which makes Old Forester and Early Times) say it is because their founders were Scottish.
Since George Dickel was German, Dickel owner Diageo made up a ludicrous story that George preferred that spelling because he believed his product to be as smooth and high in quality as the best scotch whiskies. That sounds like scotch-maker Diageo talking and it's nonsense. Scotch was virtually unknown in Tennessee in Dickel’s day.
There really is nothing more to the spelling difference between ‘whiskey’ and ‘whisky’ than a spelling difference, like ‘tire’ and ‘tyre,’ hence it’s not very important, but writers often fall all over themselves trying to use both spellings according to which national spirit they’re discussing.
You can also, if you research the subject, find all sorts of ‘explanations’ for the difference. They are all nonsense. The result of all that nonsense, unfortunately, is that many people mistakenly believe that ‘whiskey’ and ‘whisky’ are two completely different words with different meanings. Some go so far as to declare that only Scottish single malts are entitled to be called ‘whisky.’ They are wrong. They are the flat-earthers of the whiskey world.
‘Whiskey’ is one word, with one meaning and at least two acceptable spellings.
For some writers, the solution is to use ‘whisk(e)y’ as the universal term. I reject that because it is unnecessary, confusing, and because parenthesis don’t belong inside words. Since ‘whiskey’ is the only multiple-spelling word that is treated that way, such treatment suggests again that the word is unique, which it is not. Plus it is a distinction you can’t make audibly.
As this book is written by an American and published in the United States, it will use the ‘-ey’ spelling throughout, even when referring to scotch whiskey (which won’t happen very often anyway), except when a specific product name is invoked, e.g., Johnnie Walker Black Label Blended Scotch Whisky. In those cases, this book will use whatever spelling the producer uses, and also capital letters as befits a proper name.
The plural of whiskey is whiskeys. The plural of whisky is whiskies.