What is a "stray copy" of a book?

What is the meaning of "stray copy" here?

At about six I tuned to Graeco Roman mythology, lead gradually by Hawthorne's "Wonder Book" and "Tanglewood Tales", and by a stray copy of "The Odyssey" legend in Harper's Half-Hour Series.

A stray copy of a book is one left lying around, on a table perhaps, or in a drawer, not one in a bookcase standing beside other books where it belongs. It is a figure of speech (weakened by much use into the mere memory of one) in which the book is likened to an animal that has wandered from the herd or flock. It is a copy that one has come upon by accident or serendipity, not one that has been deliberately taken from the shelf.