Series books with specific storylines, like a mystery, where there is a slow build but guaranteed romance. Where you know the characters will be together in book one, and they are preferably committed by at least book 4. No triangles. They must remain the main characters - not a series about different people where former characters appear. Any genre, though romance is pretty much out by default per formula. They must be well written and engaging, preferably with a well built world that sucks you in. Examples: JD Robb, Ilona Andrew's Kate Daniels, Patricia Briggs' Mercy Thompson and Charles and Anna, Wen Spencer's Tinker, Jan Karon's Mitford Series, Elizabeth Vaughan's Warlord books. Not perfect fit, but close - Julia Spencer Fleming's Clare Fergusson mysteries (takes a bit too long for the characters to be together, and wife adds a triangle, but such good reading), Dana Stabenow's Kate Shugak (had I but known going in, I might not have started, but still the right 'type' of series). List the first book in the series only to avoid clutter.