The full expression would be “This is the book of the generation;” but this is a usual ellipse; e.g. “The vision of Isaiah,” for, ‘This is the vision.’
“Generation,” he says in the singular number, though there be many here given in succession, as it is for the sake of the one generation of Christ that the rest are here introduced.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).