What are two valid address books?

Network Prefixes in Address Books

You can specify addresses as network prefixes in the prefix/length format. For example, 203.0.113.0/24 is an acceptable address book address because it translates to a network prefix. However, 203.0.113.4/24 is not acceptable for an address book because it exceeds the subnet length of 24 bits. Everything beyond the subnet length must be entered as 0 (zero). In special scenarios, you can enter a hostname because it can use the full 32-bit address length.

https://www.junipe r.net/documentation/us/en/software/junos/security-policies/topics/topic-map/security-address-books-sets.html