I have this too, but are you sure it's POP and not just similar? The style's a bit different and the content is completely H... (POP does do a lot of covers, but I've never noted whether any are for all-out H stuff or not.)

Yes, in the h-doujinshi on the special thanks section say that both covers and page 2 is from pop. The rest of the doujinshi is from another person

Pop have some peculiar styles like the big shoes.

This is POP art yes, you cannot mistake it from anything else as none have remotly the same style as him.

The artist I've found out so far who have the closest look to POP is John Hathway but it's still quite different. Eg: http://members.jcom.home.ne.jp/ogadeng/ti.jpg

You can see (Most) of his publications here, http://doujinshi.mugimugi.org/index.php?P=LIST&T=author&ID=8 still quite a big ton missing as he publish here and there like crazy nowdays.

Uh, many doujin works mimic artists' styles (as well as their characters), and this is definitely a major variation from his usual style. The shading and coloring is so significantly different that I definitely wouldn't assume it's him just from looking at it.

May be so, but POP use a very distinct style, I'm quite into just about manga art in general as I spent many hours a day in the subject, hell I'm even the owner of the db above :P

Most artis have a own style of there own, it's true some try to mimic some styles but even those have distinct styles.

I would personally love if you know any who can mimic POP's style to the extent that it's very hard to see the difference.