My line count correctly shows the correct number of addresses on the customer record, but my log shows blank values for the fields. I am sure I have previously used something similar to this:

const _CUSTOMER = record.load({
    type: record.Type.CUSTOMER,
    id: internal_id,
    isDynamic: true,
});
const address_count = _CUSTOMER.getLineCount({
    'sublistId': 'addressbook'
});
 for (var x = 0; x < address_count; x++) {
                const address_subrecord = _CUSTOMER.getCurrentSublistSubrecord({
                    sublistId: 'addressbook',
                    fieldId: 'addressbookaddress'
                });
                _CUSTOMER.selectNewLine({
                    sublistId: 'addressbook',
                });
                const address_internal_id = address_subrecord.getValue({
                    fieldId: 'id',
                });
                log.debug({title: "address_internal_id", details: address_internal_id});
                const address_line_1 = address_subrecord.getValue({
                    fieldId: 'addr1',
                });
                const address_city = address_subrecord.getValue({
                    fieldId: 'city',
                });
            }

you dont select an existing addressbook line

I've removed this:

_CUSTOMER.selectNewLine({
                    sublistId: 'addressbook',
                });

I can now see country code, but that's all. I should see addr1, city and other bits

there are 2 parts to my comments

ah, right - thanks

or just

const c = new Customer(internal_id)
 _.forEach( c.addressbook, a => log.debug(a.addressbookaddress))