[PDF] A compendium of the comparative grammar of the Indo-European, Sanskrit, Greek, and Latin languages (August Schleicher)

I'm sharing this mostly as a historical curiosity; Schleicher was genial, but the book is a century and half old, science marches on, so it isn't exactly good source material. Still an enjoyable read if you like Historical Linguistics, as it was one of the first successful attempts to reconstruct a language based on indirect output from its child languages.