The Centaur’s Spell by Lorelei M. Hart

Apollo has been waiting for his mate for longer than anyone but his brothers have been on this planet. His brothers might think he doesn’t care about such things, but the truth is he longs for a mate so deeply that it sometimes aches. Now that his brothers are both happily mated with a growing family, it should be his turn… right?
When Walker discovers that his boss was planning to uncover magics long hidden and for a reason, he does the only thing he can think of—he reports him. With Steelwick involved, he has no choice but to follow their lead, even if it means going to live with the people he nearly betrayed.
Apollo is on edge, knowing that their new guest was connected to one of the many people wanting to uncover that which must be hidden, but then he hears his heartsong and everything changes.