Finding the library is as easy as clicking the first result of searching for “eztwain classic” in google. The link to the zip with the sources is in that site.
As you can read in that page, it’s a “lite” version of the commercial product. So if you want more features you can either buy the commercial version or extend it yourself (which is what we did). The second option implies TWAIN knowledge, so reading the TWAIN spec is a must (the twain.h file is just the interface to twain, it doesn’t explain the internal design in depth).
About the printer, as far as I know TWAIN is designed for scanners, so you should look for a different tool to access the printer functions of the Officejet J4680.