It's a novel based on the Cream Lemon episode of the same name, released in 1993 by Fujimi Shuppan.
Mostly text, with a few illustrations, none seems to be hardcore.

Just found out the real name of Kurata Yuuko, for the purists:

Inaba Mayumi (稲葉真弓)

http://ja.wikipedia.org/wiki/%E7%A8%B2%E8%91%89%E7%9C%9F%E5%BC%93

The illustrations of this book should be from Shingyouji Tatsuya.

Base +18