What were the changes in professional tennis' rules (e.g., the doubles reforms of 2006) in the currrent century? Links to the ATP rulebooks in which those rules are stated might suffice as an acceptable answer. ATP's website only has the current year rulebook.
After much research, and with the help of the Wayback Machine, I was able to compile a list of pdf's for most of the rulebooks since 2001.
To understand where we're getting these rulebooks from, we must first dive into the history of the ATP's websites, with respective rulebooks. The chronology of the internet presence of what is currently the "ATP World Tour" goes a little something like this:
Chronology of "ATP World Tour" Website
1995 - "The ATP Tour launches its first ATP website, ATP Online..." 1 Unfortunately, not much is known about this site, and it doesn't appear in any search results I performed. As a result, no rulebooks from this time period have been provided
2001 - "A newly launched website, ATPtennis.com, highlights the new tournament structure. ATP Properties forms, instigating a more commercial focus and new marketing, licensing and broadcasting opportunities." 2 Rulebooks referenced from 2001 and up to 2008 will be from this site and not the current ATP World Tour site.
2008 to 2018 - The ATP World Tour website as we know it today first came into existence in 2008 and has largely kept the same form even into today. The majority of rulebooks come from this site's archives.
Without further adieu, here are the rulebooks up until 2001:
ATP World Tour Rulebooks "2001-2017"
2012 ATP World Tour Rulebook : Click on "rulebook" and download the file (only way to access)
2011 ATP World Tour Rulebook : Click on "rulebook" and download the file (only way to access)
2009 ATP World Tour Rulebook : The page states the rule changes at the bottom from 2008 for the 2009 season, which is what OP is looking for anyway
2008 ATP World Tour Rulebook : The "rulebook" link is unfortunately inaccessible, but I will update my answer if I am able to find the 2008 edition
2007 ATP World Tour Rulebook : The "rulebook" link is unfortunately inaccessible, but I will update my answer if I am able to find the 2007 edition
Here are the specific changes from the 2002 to 2003 rulebook
2001 ATP World Tour Rulebook* : I was unable to find the rulebook on the site, and do not believe it exists anywhere online
A special thank you to Martin Sleziak who helped provide the 2010 and 2014 editions.