As far as I know, there is currently no way to retrieve all address book entries via RPC.
This functionality can be added by slightly modifying the getaddressesbylabel and making the label parameter optional.
PR:
./src/bitcoin-cli -regtest getaddressesbylabel
{
"bcrt1q23d9qmeqrmdfazp79e5xavpgrzqmmtjf7k2p7u": {
"label": "label03",
"purpose": "receive"
},
"bcrt1pynqgl3mh8qmwj496nfg6ucxd8jw0wz3rz7jtf4s4v6e9zv6a4cnqdcl79k": {
"label": "label02",
"purpose": "receive"
},
"bcrt1pty99hxsx7trd53ul7lhl4ee82ggqslyq54mg5mte5wv8nfu7xxfqcf0pzz": {
"label": "label01",
"purpose": "receive"
}
}
On master branch, it is not possible to run that RPC without passing a label parameter.