This is book number 2 in the Ologies series.
Daniel Cook and his sister, Beatrice, are thrilled to be studying dragonology with Dr. Ernest Drake--they're even awaiting the hatching of a dragon's egg in their care But all is not well: there are rumors of a deadly plague affecting dragons around the world. Could dark dragonologist Alexandra Gorynytchka be behind the mysterious illness? The siblings soon discover that the secret to curing the dragons might be found in the Liber Draconis, a diary written by a wise dragon ages ago--if only they could decipher it So they set off (with their mischievous dragon chick in tow) on the Dragon Express, riding one dragon after another in a race to bring Liber Draconis--and a cure--to Dr. Drake before it's too late.
