Taking a break from her two long-running series (Agatha Raisin and Hamish Macbeth), Beaton introduces a pair of engaging misfits whose struggles to cope with overbearing families and overweening circumstances teach them, eventually, to rely on each other. When diffident fortyish virgin Fellworth Dolphin's mother dies, he finds himself surprisingly relieved and freed from a bondage he was only partially aware of. That's nowhere near the astonishment he feels when he learns that, in spite of their penurious existence, he's heir to a large sum. In a moment of panic, when it seems an aunt might assume the tyrannical role his mother once played, Fell pretends an engagement to mousy waitress Maggie Partlett. In fits and starts, Maggie and Fell begin their separate transformations--she to a swan, he to a drake. One catalyst is the money and its questionable provenance--perhaps the result of an infamous train robbery that occurred many years ago and that Fell's father might have been involved with. The other is the transformation wrought by their shared investigation and their shared lives, as Maggie falls in with the pretended engagement for her own purposes. Various relatives and villains attempt to derail the couple as they journey, but there's never a question of where the author is taking her odd couple, and never a doubt they will arrive safely. The trip will delight fans of either of Beaton's other series. (Mar. 21) FYI: Beaton's latest Hamish Macbeth mystery, Death of a Dustman, will be published on Mar. 6 (see Forecasts, Dec. 18, 2000).