The Astronomy Book is a book that can be found on a lone table in the Slaughter Gulch Bank.
The book's full name is Practical Astronomy, but even after reading it, it's still credited as "An astronomy book".
Function
The astronomy book describes the Corona Borealis constellation, which constitutes of five stars, one of them known as the Pearl. It gives a logical hint saying that the Pearl must be used in the Bank Vault, which has four stars stamped in the vault's door.
Inscription
Practical Astronomy
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by C.N. Rokonen
Framed by Hercules and the
Serpent, lies the Corona Boreallis
constellation. 179 degrees squared wide,
Corona Borealis counts 20 stars of an
absolute magnitude superior to 6, and
therefore visible to the naked eye.
An unpracticed eye may only
distinguish 5, forming a circular arc.
Beneath this cricular arc lies the
brightest star in the constellation:
Corona Alpha, known as the Pearl