In the United States, the standard for most books is to use a serif font for the text of a book. Serif fonts are ones that have little edges on the letters. The theory is that those edges help to guide the eyes along the line so the person can read more smoothly. Look for example at the base of the F in Four. Here is an example with Times Roman.
In comparison, sans serif is used less frequently as the text for a book. It's often used for headlines and titles. This is Arial. Again, check out the base of the F in Four.
The third main option would be monospace but this is much harder for the eye to read and is rarely used. This is how typewriters used to create letters, with each one having an equal space with the others. An author friend of mine used this for his science fiction story to give it an artificial feel. This one is Courier.
I'll also note that technically the way a given set of letters looks is its typeface. Times Roman is a typeface. Arial is a typeface. It's only when you have a specific combination of typeface, size, weight, and style (bold, italics, etc.) that the end result is called a specific font. However, in our modern world many people use the two words interchangeably and many don't know what a typeface is.
Keep in mind that many ebook readers allow the user to choose a font and size that is easiest for them to read. So while you might make the choices for your paperback version, whatever you set for your ebook version is merely a suggestion.
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