It was shown as round. It was well understood for centuries prior to space travel that the Earth was round, and globes were very common and widely in use especially from the 17th century onwards (and were ubiquitous from the 19th century). In books the Earth would have been reproduced as a map or as a picture or print of a globe or perhaps (in an astronomical context) as a hypothetical picture from space.

These were pretty accurate, depending on the era and quality of manufacture of course, because they were based on extensive land surveys (and later aerial photography). By, say, the 1950s a globe of the Earth was a reasonably accurate representation of the land vs. ocean layout of the planet. They did miss the mark in a couple of key areas though. One is in the colors and textures of the land and ocean, before satellite imagery humans couldn't really appreciate the nuances and details of the appearance of the surface of the Earth from a distance. Another is the seasonal differences in appearance of the Earth's surface. Maps and globes portray a static view of the Earth, but the reality is that even the land of the Earth changes throughout the year due to the seasons both in terms of changes to vegetation and snow/ice cover. Some maps/globes do things like omit the polar ice caps entirely, for example, others just sort of pencil in a typical representation. In a similar vein almost all representations of the Earth prior to the advent of spaceflight don't portray cloud cover even remotely correctly. On a typical day fully 2/3 of the Earth is covered by clouds, and these have complicated structure at very large scales, which was not generally (or at least commonly/sufficiently) appreciated before spaceflight.

The result is that attempts to portray the Earth (or Earth-like planets) from space in pre-spaceflight works tend to be notably deficient even though they might get some things like the shapes of continents right.

One of the most famous artists of space scenes before and in the early years of spaceflight was Chesley Bonestell, who put a lot of work into achieving as much accuracy as was possible. Here are two examples of his work showing the Earth from space this one of a space station on orbit and this one showing a launch from Florida. These are both very good efforts but both lack some of the fundamental character of true images of Earth from space. For example, the color and texture of the landscape is too uniform. Also, there are only some small, wispy clouds. Which could be a stylistic choice but is still fairly unrepresentative for Earth. You can also see a representation of Earth in some of the original Star Trek episodes (such as this one from 1968), and in almost every case the representation of a planet looks like a globe without labels, completely lacking in weather. Though even in the late 1960s some portrayals of Earth from space were more accurate (such as in 2001: A Space Odyssey). Though it took until the 1970s for media portrayals of planets from space to attain more realism (due mostly to greater exposure to imagery of the Earth from space).