I checked the field guide and it appeared on the page just before the picture of the much more common Great Egret, a very similar bird. Read the text in the field-guide, don't just look at the pictures. Be wary of basing your identification on plumage alone. Plumage is probably the most variable feature of a bird and if you're not paying attention to other things, like the shape and color of the bill, behavior, vocalizations, leg color and eye rings, you may be making a mistake. I once had a call from a woman who had five American Bitterns roosting in a tree.
Rare birds show up outside of their range all of the time.
Make sure you can legitimately eliminate the more likely local birds. In the Chicago area, you're much more likely to get a Summer Tanager than a Hepatic Tanager. So read the field guide carefully, and rule out the Summer Tanager, before you convince yourself you've found an Hepatic Tanager. Pay particular attention to the female and immature plumages. These will often confuse new birders who don't realize that the female Western Tanager they think they have is really a female Scarlet Tanager.