According to Wikipedia:
The word [frontispiece] comes from the French frontispice, which derives from the late Latin frontispicium, composed of the Latin frons (‘forehead’) and specere (‘to look at’). […] In English, it was originally used as an architectural term, referring to the decorative facade of a building. In the 17th century, in other languages […], the term came to refer to the title page of a book, which at the time was often decorated with intricate engravings that borrowed stylistic elements from architecture, such as columns and pediments. Over the course of the 17th century, the title page of a book came to be accompanied by an illustration on the facing page […], so that in English the term took on the meaning it retains today as early as 1682. By then, the English spelling had also morphed, by way of folk etymology, from ‘frontispice’ to ‘frontispiece’[…]
World Wide Words has more about the history of frontispiece, and it elaborates on a line referring to divination that I elided from the abovequoted Wikipedia piece. WWW says:
You might think that [frontispicium] referred to the forehead being a clearly visible part of a person’s head, but it seems instead to have been connected with [a Classical divination method that’s also known as] metoposcopy […]; it’s the art of telling people’s character or fortune from their foreheads[…]. The association of ideas seems to be that the façade of a building is as expressive as a person’s forehead.