Traeger Grills are wonderful. This little cookbook title, however, is problematic.
An apostrophe can be used to show a letter is intentionally deleted. A typical example is when we depict a rural or casual pronunciation of the "ing" at the end of a verb. Example: "I'm singin' in the rain."
The good folks at Traeger apparantly wanted to send a rural/casual message, but they did not want to go so far as to delete the "g" at the end of "Traegering."
Hence, this odd case of an apostrophe that has no purpose whatsoever. It's just smoke.