As a non-historian beginning some researches into local history, I have found this small volume invaluable. Grappling with Gregorian and Julian dates, never mind 'regnal years' dating, has never been my favourite pastime, but this resource cuts straight through to the chase. I don't use it every day, but I certainly feel satisfied that were I have to a problem with understanding a historical date, I'd have a 99% chance of solving it with this. Obviously, unlike the previous reviewer, my needs are not highly demanding or academic, but I think a professional would know where to go to get their questions answered. I would be surprised if enthusiastic amateur local/family historians weren't fully satisfied by Cheney - at worst, I would have thought there should be one copy per local history society!
Although this is a reference book rather than a reading book, it's also fun dipping in and finding out arcane factoids to amaze and delight your friends and family!