There's no .lit reader for Macs. It's a proprietary format owned and strictly controlled by Microsoft. Nevertheless, there's a way to convert some .lit files to a readable format using a program called Convert LIT. Conver LIT can be a pain to use at the command line, so I wrote a quick ApplScript droplet for it.
In order for this to work, the Convert LIT binary must be in the invisible /bin folder on your boot drive, which will ordinarily require an admin password. To use, first copy my code. Then open the Script Editor and paste in the code, and save it as an application.
From that point on, you should be able to drag a .lit file onto your new application and have it "exploded" into a new folder containing the component JPEG images, HTML files, and Open eBook files. Please use the code to convert files that you legitimately own.
[robg adds: Please note that the compiled binary from the above site is named with a semi-obvious acronym. If I were to use that acronym here, however, it might cause macosxhints to get screened by some automated internet filtering programs, so I've chosen to refer to the program by its full name. In testing this hint, I found another .lit converter, the Open Convert-.LIT tool. I have no idea what, if any, relationship the two apps have, other than they seem to share the same four-letter acronym.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060716020829624