Thanks for the suggestions.
I’m still researching and I think something more CRM instead of a basic address book is needed. We need more detail than just basic contact info; who is a regular maintenance customer, who has spent over x amount of dollars with us and is therefore more lucrative and loyal. A way to tag them, pull up the tags quickly to make a mailing list, and then send out invites to call early for appointments.
(And with tagging, maybe a way to do something special like x dollars off the next service for referrals or some other customer loyalty program.)