Murder is on the menu... And if you have an appetite and a taste for an intriguing murder puzzle - then try sampling Pool of Deceit.      As Alistair Fortnam imparts. "Finding out if someone is telling a lie is only part of the equation, some might say the easy part. The difficult part is finding out why they are lying."      Pool of Deceit features Alistair Fortnam of New Scotland Yard, a brilliant detective who has extensive training in Behavioural Sciences. He's had a gilded upbringing, a car-wreck of a marriage (now many miles behind him) and an upwardly-rising career.      We find him in Florida on assignment and leading an investigation into a sudden death at the Blue Heron Hotel on Anna Maria Island.      Diane Lewis, a top sales manager for a pharmaceutical company dies after eating a calorie-loaded hamburger at the hotel's swimming pool - although it was not cholesterol induced heart attack that killed her, but poisoning of a kind.         After Fortnam interviews the thirteen suspects, the investigation quickly becomes indigestible to him as well.      Interestingly, some witnesses have quite different motives as why they may have wished Diane Lewis harm, although identifying their secrets and the guilty party is as tricky as untangling an old set of Christmas lights in a dark attic. Fortnam attempts to sort it all out, and make the right connections.