Bain's book—the product of a fourteen-year effort—is the authoritative account and will probably remain so for a long time. If you want the whole story, it's here, 700 wonderfully written and meticulously researched pages. What the transcontinental line was to railroad construction, Bain's book is to railroad history.
For those who don't want to hunker down with Bain's epic, there's the Ambrose version, weighing in at about half the length. Ambrose gathers the story as presented by many secondary sources and peppers it with great quotes and biographical detail. The upside is that it's a breezy read; the downside is that Ambrose's tendency to cheerlead for the indomitable American spirit leads him to paint nearly everyone involved as a larger-than-life-type character and to come off as rather uncritical of corruption and excess.
The building of the transcontinental line across the vast mountains, plains, and deserts of the West was, undoubtedly, a spectacle to behold, and this is the book that puts you there. Russell had been the official photographer of the U.S. Military Railroad during the Civil War, and in 1868, he set out to document the Union Pacific's journey to Utah. He came back with a collection of images for the ages. The black and white photographs are stunning and capture the grandeur of the project and the places in a way that words are still struggling to match.
If you wanted to wade into the quagmire of corrupt railroad finance—and, admittedly, that's a pretty big 'if'—White's article would be a good place to begin. He details not only the practices involved, but also the historical culture in which they should be understood. If you can get through it, White leaves you with solid idea of how the financiers manipulated information to play investors, inspectors, the media, and even each other—and turned the wild profits that they often did.