You are conflating two factors. Commercial use is about what the copier is doing with it. If you download a copy for information related to your non-commercial dives, then it's not commercial. Downloading it to print and re-sell obviously would be. Other uses might fall in the middle.Taking a pirated complete PDF copy instead of buying an available hardcopy (at an ever increasing price point due to scarcity) seems to clearly fall into the commercial use definition?
The price of copies on the secondary market is relevant to another factor: "the effect upon the potential market for or value of the copyrighted work". But mitigating against that is the fact that by allowing the books to go out of print, the publisher has made its own clear determination that the potential market for and value of the work is limited. Especially in today's world where the availability of print on demand services and electronic distribution has made the cost of keeping an existing work for sale trivially low.
For anyone who is really interested in this, there's tons of info out there about making fair use determinations. Here's a trio of links to get you started. The first is a fairly compact summary which is good enough for a general understanding.
Copyright and Fair Use
The guide set forth below is available in PDF here: Copyright and Fair Use: A Guide for the Harvard Community. You may also read frequently asked questions concerning copyright and privacy for more information. Copyright and Fair Use: A Guide for the Harvard Community CONTENTS Basics of...
ogc.harvard.edu
U.S. Copyright Office Fair Use Index
The goal of the Index is to make the principles and application of fair use more accessible and understandable to the public by presenting a searchable database of court opinions, including by category and type of use (e.g., music, internet/digitization, parody).
www.copyright.gov