Pliny: Naturalis historia – Natural History
Rackham's Loeb edition of the NH is available in two partial formats: the entire translation without the Latin text is provided in a HTML version by, while most but not all volumes (including Latin text) are available as scanned books at the Internet Archive. The original Latin text, in Mayhoff's Teubner edition, is available as HTML files from Lacus Curtius. 

The Perseus Project's HTML English version uses Bostock and Riley's translation of the 1850s; this has the advantage of giving place-names in their ancient form (the Loeb gives the modern rather than the ancient names for many places). 

Complications arise from the fact that two distinct section / chapter numbering systems exist for the NH. It is, therefore, not possible to relate Giannini's cross-references directly to either the Masseiana or the Perseus versions of the text, since he uses one system while the two translations use the other. The printed and scanned Loeb versions and Mayhoff's text on Lacus Curtius use both systems.

Citations of Pliny given here use the Bostock and Riley translation on Perseus, for ease of reference regarding place-names, and use both numbering systems -- e.g. II.iv.10 rather than II.iv (Masseiana / Perseus) or II.10 (Giannini).

A spreadsheet showing the relationship between the different numbering systems will be attached when fully compiled (for those with Latin or who are using Rackham, the Loeb and the Lacus Curtius versions, unlike Giannini, show both of the alternative standard section/chapter numbers). 

Pliny, trans. Rackham, Jones et al (1938–1962) Natural History (10 Vols). Cambridge, Mass; London: Harvard University Press; Heinemann. [HTML]
Pliny, trans. Rackham, Jones et al (1938–1962) Natural History (10 Vols). Cambridge, Mass; London: Harvard University Press; Heinemann. [Scanned]
Vol I (Bks I, II)   2 (III–VII)   3 (VIII–XI)   4 (XII–XVI)   5 (XVII–XIX)   6 (XX–XXIII)   7 (XXIV–XXVII)   8 (XXVIII–XXXII)

 Pliny, ed. Mayhoff (1875–1906) Naturalis Historia (5 vols). Leipzig: Teubner 

Pliny, trans. Bostock and Riley (1855–1857) The Natural History (6 vols). London: Henry G. Bohn.