If you're interested in an unabridged audio edition, I recommend that narrated by Geoffrey Matthews over the David Case version. Matthews, to my way of thinking, has a better voice, produces more distinct characters, and brings the text more vividly to life. (Naturally, his recording seems to be harder to find just now.) David Case is OK - he sounds exactly like the narrator for Aird's 'Cause and Effects' - but I was spoilt by hearing Matthews' reading first.
"The Secret of Father Brown" - In this prologue, Father Brown has come to visit Flambeau, who has long since retired to a castle in Spain. Another visitor asks Father Brown for the secret of how he solves all his cases - and gets a startling answer. The epilogue at the end of the book is supposedly the end of the same evening (all the stories in between having been produced as examples). Don't worry, the narrative style is the same as usual; the prologue and epilogue are just here to tie all the stories together.
The key to coping with Chesterton's stories is to remember the dictum of Dorothy L. Sayers' Lord Peter: "When you've got how, you've got who." If you go haring off after motive in a Chesterton story, all I can wish you is luck; you'll need it. They're good stories, with lovely use of language and settings, but weird things happen for weirder reasons, sometimes. Just sit back and enjoy, and don't worry about whether anybody could *really* hope to get away with some of these crimes. Some stories have multiple crimes, where one crime is committed because of another. If you feel sympathy for some of these 'second' criminals, you might also like to try Chesterton's _The Club of Queer Trades_, even though Father Brown doesn't appear there.
"The Mirror of the Magistrate" - Agatha Christie's Poirot once asked Hastings to mention 'chocolate box' to him if he ever needed to be humbled with a reminder of failure. This case is the closest that Father Brown came to that - he refers to this case in later stories whenever his terse comments divert the authorities in the wrong direction in an investigation. I take comfort that *somebody* felt guilty about all those red herrings... :)
"The Man with Two Beards" - This case is sometimes referred to as the Moonshine murder. Michael Moonshine is a legendary burglar, who "stunned people - and bound and gagged them," but who made it a point of honour never to kill anyone. Now he's apparently in the neighbourhood - but someone died during this robbery. What really happened? (Incidentally, for Moonshine-style burglary, let me recommend Looking Glass Studios' game _Thief_.)
"The Song of the Flying Fish" - Locked-room theft (that is, a locked-room mystery which is a theft rather than the traditional murder). The rich man's favourite toy, an antique glass bowl of solid gold fish, gave him his favourite joke when meeting new people: "Have you seen my gold fish?" Now somebody, upon seeing them, has caused them to disappear.
"The Actor and the Alibi" - Locked-room murder. How was the theater manager murdered in his locked office, especially when most of the company was on stage for an undress rehearsal? The only member of the company who wasn't in view of witnesses - a hot-tempered Italian actress dissatisfied with her part - had locked herself in her dressing-room, hence Father Brown being called in to reason with his parishioner. (On being asked whether to break the door down, the priest advises against it, contrasting her with a certain broody metaphysical type; the other half of the comparison can be found in Chesterton's "A Tall Story" in _The Paradoxes of Mr. Pond_.)
"The Vanishing of Vaudrey" - Locked-room disappearance, more or less. The local squire disappeared in the middle of the morning in the tiny village near his home. Father Brown begins by accompanying Vaudrey's secretary in the search, and hears his troubles while picking up background information on the setting and characters.
"The Worst Crime in the World" - Father Brown accompanied his friend Granby to Sir John Musgrave's castle, but not to assist in finding out if Sir John's son was a good credit risk. Young Musgrave needs money because he wants to get married to Father Brown's niece - who isn't quite sure about him. Unfortunately, while setting Granby's mind at rest, Sir John made a cryptic pronouncement about his son's character...
"The Red Moon of Meru" - Again, a theft rather than a murder, and leaves an impression much like that of 'The Song of the Flying Fish' (see above). This time, a mystic has apparently made a ruby vanish - the best thing that ever happened to his reputation, in terms of psychic phenomena.
"The Chief Mourner of Marne" - The Marquis of Marne dropped all his old friends and left England many years ago, upon the sudden death of his best friend and idol, his cousin Maurice Mair. Even upon his return, he has shut himself up and appears to be obsessed with religion - his old friends often bewail the 'vampires' he's taken up with. Enough so that one of them now complains to Father Brown, who isn't about to hear his religion slandered...
"The Secret of Flambeau" - Returning to the scene of the prologue, the American visitor questions the wisdom of Father Brown's charity with criminals, to be countered by a rather startling defense from Flambeau. (See the first few stories in _The Innocence of Father Brown_ if you're not already familiar with Flambeau's history.)