Περὶ ὕλης ἰατρικῆς, or "De Materia Medica", an early pharmacopoeia, composed over 20 years by a Roman army medic. Or another composed later, Περὶ φυτῶν ἱστορία, "Peri Phyton Historia", ten volumes just on plants, written as notes for lectures to students. Most texts written by philosophers of the day would also have included a lot more analysis.

Of course, Pliny referred to the first book and others in writing his Natural History, but it ended up as a sort of Dummy's Guide to Natural History, trying to compose a really wide subject matter into a few books over 2 years, and obviously getting a lot wrong in the process. It's fair to say nobody would have taken this seriously if not for his name.

I haven't read Pliny (though I've read that he had much lower epistemic standards than e.g. Aristotle), but here's a perhaps similar case: I have a volume from a "great classics" series with the surviving works of Archimedes, Apollonius, and Nicomachus -- three ancient "mathematicians". I put that word in quotes because though the first two fully merit it, the latter work is embarrassing stuff, hardly more than numerology. Nicomachus lived around Pliny's time, several centuries after the heyday of Greek/Hellenistic science.

This tangent's in response to all the downvotes for peterwwillis's opinion -- it seems reasonable to me, except maybe he should've said the standards of pre-Roman Greece.