Good books specifically about Libya are hard to find outside specialist libraries, but a few are worth tracking down.
Travels in Arabia Deserta by Charles Doughty (1888) covers the wider region and established the template for serious desert travel writing.
The Libyan Desert by W.J. Harding King (1903) is a classic account of Saharan exploration in what is now southeastern Libya. Available in reprint editions.
Libya: The History of Gaddafi's Pariah State by Ronald Bruce St John is a solid political history covering the colonial period through the revolution. Readable and well-researched.
The Return by Hisham Matar (2016) — a Libyan-British novelist returns to Benghazi after the revolution to search for his father, disappeared by the Gaddafi regime in the 1990s. It won the Pulitzer Prize and is the most important book written about modern Libya. Essential reading before any visit.
Libya: From Colony to Revolution by Ronald Bruce St John (revised edition 2012) gives the clearest account of Libyan history from the Italian conquest to the fall of Gaddafi.
Alone Across the Sahara by Jean-Claude Guilbert — one of several accounts of trans-Saharan travel that pass through Libyan territory. Useful for the Fezzan region.
For the Roman sites, the guidebook Libya: The Bradt Travel Guide by Daniel McLaughlin (last updated 2008) was the standard reference for pre-revolution travel. It's now outdated on practical matters but still useful for historical and archaeological background on Leptis Magna, Sabratha, and Cyrene.