We are reading a book that contains an account of early space flights. what kind of of pronoun is that in the sentence

There are two pronouns in this sentence - the word 'we,' and the word 'that.' Since you didn't indicate which one it is you need to determine the type of, I will just explain both of them.
The pronoun 'we' is a personal pronoun, meaning that you would use this word to refer to yourself and others. Some other personal pronouns are: I, you, he, they, etc.
The other pronoun in the sentence is 'that,' and it is a relative pronoun. This means that it connects two parts of a sentence by referring to a noun in the first one. Here, the word 'that,' which can be substituted with the pronoun 'which' and have the same meaning, refers to the noun 'book.'
Indefinite pronouns are those such as somebody, anyone, etc. Interrogative pronouns are used in questions - which, who, etc. Demonstrative pronouns are used when you are pointing to something - Give me that book.