Well, I'm not Ian but, having read a goodly chunk of de Camp's fantasy over the years, I'll take a go at this one.
Works that were seminal? Perhaps. (I'll get back to this in a moment.)
Works that are famous? Most definitely.
The Goblin Tower? Well, here's Wiki's entry on the series of which it is the first tale:
Novarian series - Wikipedia, the free encyclopedia
As for seminal works... I'd say that his works with Fletcher Pratt, such as the Harold Shea stories, if not seminal were not too far from that, as they've been something of an influence on a lot of the more humorous fantasy writers since. (They are also, despite a somewhat awkward opening chapter to the first, a rather good set of stories in themselves.)
Harold Shea (fictional series) - Wikipedia, the free encyclopedia
The Carnelian Cube, though much less successful, is worth a read if you can find it... though only
just, at times. The
Gavagan's Bar series did something of the same sort for sf, along with Clarke's
Tales from the White Hart.
Lest Darkness Fall, though blatantly in the mode of Twain's
A Connecticut Yankee in King Arthur's Court, is (with some justice) considered a classic in the sff field. His Pusâdian series is also well worth reading, though the entire set (to my knowledge) has not yet been gathered together:
Pusadian series - Wikipedia, the free encyclopedia
He also did quite a few good sf stories over the years, though perhaps the best-known of them today would be his Viagens Interplanetarias:
Viagens Interplanetarias - Wikipedia, the free encyclopedia
Amusing, but not entirely to my taste, I must admit.
If you go for de Camp's work, keep in mind that he was largely (though not exclusively) an ironist rather than a straightforward tale-teller. His work is full of humor, satire, and wry observations on humanity's foibles. I'd say his major fault as a fiction writer is an annoying habit of repeating (almost verbatim) passages from other stories now and again without any seeming reason other than to avoid rewriting a similar scene. This can be jarring if you read much of his work at a go, but spaced out over a longer period it's unlikely to be a problem.
He also wrote a lot of nonfiction, such as a book on the so-called "Scopes Monkey Trial" and a (somewhat controversial) biography of H. P. Lovecraft, as well as one on Robert E. Howard. There was also a series of articles on famous fantasy writers, gathered together as
Literary Swordsmen and Sorcerers:
Literary Swordsmen and Sorcerers - Wikipedia, the free encyclopedia
He also wrote a fair number of books criticizing pseudo-history and pseudoscience, several of which are very entertaining and informative.
Lands Beyond, which he wrote with Willy Ley on the subject of the reality of fabulous lands of myth, won an International Fantasy Award back in 1953, and remains a very enjoyable read today. (It was reprinted back in 1993, I believe, and is fairly easy to find....)