Well, I have to agree that Clancy is entertainment, and I enjoyed his books when he started, but they became crap too fast. Characters are cardboard, way too simple minded conservativism, overzealousness in tech.
I liked Ludlum too, but it has the same problem- eventually it turns to crap. Ludlum is all about super conspiracies, tough language, and paranoia.
McClean's Guns of Navarone is also a favorite adventure yarn, but he too suffers the same fate- stories become too predictable. Compare- When Eagles Dare and Force 10 from Navarone and the stories look alike.
This is the problem with recycled genre fiction- after awhile it turns to crap. Why? because its too much formula. When it becomes just mass produced crap than it stops being entertaining because its not fresh or interesting.
But I also think that "good literature" is also a sub genre in itself even if it does not regard itself as such.
I like to read horror (and yes, most of it is crap). I think Koontz's problem is that he has to avoid becoming too manufactured. King can be great but sometimes he just rambles on for hundreds of pointless pages.
But then why are Jules Verne, HG Wells, Poe, Stoker and Mary Shelly, and increasingly Lovecraft, considered to be 'literature' and genre is considered to be crap? These horror/sci-fi writers were not that signficant in their lives but became "literature" only after the writers had long since died. I think a few of Kings novels will probably be seen a classics sometime in the future.
So I think we need to be careful with entertainment vs. literature.
Right now on my plate for fiction I have- mostly because I am thinking about detective fiction and thrillers, but also the noir genre-
Conrad's the Secret Agent- so far so good.
King's Wolves of Calla
Thompson- The Killer in Me-
Hammett- Maltese Falcon and Red Harvast
Lots of Patricial Highsmith
Richard Stark
James Ellroy
and the Zanzibar Chest (gift from wife).
But that's only what's on the shelf waiting to be read.
For those interested in crime noir, check out-
Crime Novels : American Noir of the 1950s : The Killer Inside Me / The Talented Mr. Ripley / Pick-up / Down There / The Real Cool Killers
That said, I think you should be selective and wide ranging in your reading. Read "good" fiction but also don't shy away from entertaining fiction as well. Some of it is quite fun and good.
For example here's a piece from the beginning of the "Killer Inside of Me"in which the main character- the local sherriff who has a sociopathic side that he is trying to contain meets up with a citizen who thanks him for helping out his son. The story is told by the sherriff in the first person-
"Only you," he said, "Because you are good, you make others so." He was all ready to sign off with that, but I wasn't. I leaned an elbow on the counter, crossed one foot behind the other, and took a long slow drag on my cigar. I liked the guy- as much as I like most people, anyway- but he was too good to let go. Polite, intelligent: guys like that are my meat.
A wonderful bit. But probably you will have to read the story.
Let's see favorite stories (which transcend both "good literature" and entertainment)-
Robert Louis Stevenson- Dr. Jeckell and Mr Hyde.
Jack London- The Seawolf
Raymond Chandler- The Big Sleep.
Conrad's - Lord Jim
Dumas- The Three Muskateers
Eco- Foucault Pendulum
Lovecraft- the Dunwich Horrow, Call of Cythulu
McMurphy- Lonesome Dove
I agree with Kharn- Watership Down is a great read.
Theroux- the Mosquito Coast
More, but I can't think of them right now.