''Μας'' here is an indirect object pronoun. (They are reading a book to us)

The direct object pronoun for 1st person singular just happens to be the same.

''Ένα βιβλίο'' could be replaced with a pronoun, if it is a specific book that has been mentioned before or the speakers know about it in advance. Here, however, you should use a direct object pronoun since it replaces the direct object of the sentence:

''Μας διαβάζουν το βιβλίο'' --- ''Μας το διαβάζουν''

Here is a list of both Direct and Indirect Object Pronouns:

http://blogs.transparent.com/greek/personal-pronouns-direct-indirect-object/