Generally speaking, it’s not terribly difficult today to find a book you’d like to read.  Perhaps it’s on the shelves at your quaint local bookseller, or somewhere among the discounted cookbooks and self-help tomes at the cavernous warehouse of a major retailer.  You could check your local library, and certainly order it from another branch if necessary.  Or you could scour Google, to see if you might locate a scanned copy uploaded, licitly or illicitly, by some anonymous bibliophile.
In antiquity, however, a book was a much rarer and more precious possession, with the hand-copying and pumice-smoothing and all that.  Fewer physical books produced, of course, meant that fewer would survive the ravages of time, and I’m sure we’ve all pined for a certain lost work (Claudius’ book on dice games)… while wishing an angry band of Vandals had torched some of those we’re stuck with (I’m looking at you, Caesar).  Although the complete texts of so many ancient books are long gone, at least we find some helpful remnants here and there — a quotation in a grammarian because the lost author used an odd word for “eel”; a papyrus scrap from the Egyptian desert; a charred scroll from Pompeii; a jejune summary from Constantinople.  With these fragments, scholars can attempt to reconstruct the original text, guessing at its content, its characters, and perhaps even where in this original the surviving portions belonged.
To illustrate the challenges which one might face in dealing with a fragmentary text, I have my students attempt to reconstruct a modern film from fragments of its dialogue.  Using a resource like IMDb, after writing up a list of movies which my students are unlikely to have seen, I select ten chunks of dialogue from each, preferring passages which can be readily quoted by any fan of that film.  Each quote is placed onto a separate note-card, and then all ten cards are placed into an envelope.  Breaking the class into groups of three or four students, each group is given an envelope containing quotes from a different film.  The students must then (a) attempt to summarize the plot of the film based solely on the fragments, (b) describe who they think the characters speaking these lines might be, and (c) place the passages into the order they think they might appear in the film.  Some groups will play it safe and make no inferences not completely supported by the text, whereas others will get… creative; I’ve received, from such groups, some very interesting approximations of the plots of The Big Lebowski and Pulp Fiction!