The Baltimore conspiracy to assassinate Lincoln before he reached Washington for his inauguration in 1861, while less well known than the plot which succeeded four years later, has nevertheless been told in print a good many times before now. Mr. Potter has not added much, other than a mostly unsuccessful ""you-are-there"" quality mainly conveyed by a tiresome use of present tense, but he has put it all together in a way which is quite clear and easy to follow. The tale itself--and it is a classic bit of cloak-and-dagger-ism-is-does the rest. The hero is that enigmatic little Scotchman, Allan Pinkerton, who with his host of colorful operatives uncovered the plan just in time and thwarted it by conveying the President- elect through the hostile city incognito several hours ahead of schedule. Had this counterplot failed--well, subsequent history would have been considerably altered, to say the least. With characterization which is succinct and believable if rather stock, and with a gift for including most of the right little details for continued interest and understanding, Mr. Potter and Mr. Pinkerton get us to the capital safe and sound and (or should one say but?) scarcely out of breath.