I have written before about my recommendations for essential genealogy books and today one of them is on sale 40% off. If you do not already own Evidence Explained: Citing History Sources from Artifacts to Cyberspace by Elizabeth Shown Mills, the publisher has the second revised edition for $35.99 down from $59.99.
In addition to this book being the best reference available for genealogical citations, it also contains an excellent chapter on evidence analysis. I recommend that this book be on every genealogist's desk as a handy reference.
Here is the link to take advantage of the discount from Genealogical Publishing Company: