Where did historic fiction writers find those strange Roman recipes? Mostly from Apicius!
M. Gavius Apicius was a 1st century A.D. gourmet who, among other things, cooked at his own banquets. He is supposed to have written two books: De condituriis is all about sauces. The second one appears to have acquired additional recipes over the ages during which it was copied, and became what we now know as de re coquinaria, in 10 books.
Amended December 14, 2006: It seems that the above Apicius is actually not the author of the book which, from the language style has been compiled in the late 4th or early 5th centuryAD. No one really seems to know who the author or authors were.
Wikipedia has an extensive article on Apicius (as usual handle with care), and according to this, if the text is to be believed, it was Apicius who invented foie gras, not the French.
Translations in print have mixed reviews.
Yahoo! has a group dedicated to Apicius: Apicius · This list is for sharing experiences in Antique Roman cookery.