Far Away and Long Ago by W.H. Hudson (1918). Hudson grew up on the Argentine pampas and in the northeast, and this memoir of his boyhood captures the natural world of the region — birds, animals, landscape — with extraordinary vividness. The finest nature writing to come out of Argentina, and essential reading for anyone travelling through the Paraná basin.
The Voyage of the Beagle by Charles Darwin (1839). Darwin spent considerable time in Argentina on the Beagle's survey voyage, and his observations of the landscape, wildlife, and people remain fresh. The broader South American context it provides is useful for understanding the continent.
In Patagonia by Bruce Chatwin (1977). Chatwin's account of his journey through Argentine Patagonia established a template for a certain kind of travel writing. The Argentina it describes is remote and eccentric and entirely absorbing.
Birds of the High Andes or a Neotropical field guide (various authors). The park's 400-plus bird species make a good field guide worth carrying. Ridgely and Tudor's Birds of South America is the scholarly standard; Ber van Perlo's illustrated guides are more portable for field use. Either will transform the circuit walks considerably.