Diamond Brothers

Series of detective/comedy novels by Anthony Horowitz, featuring would-be private eye Tim Diamond (real name Herbert Simple) and his kid brother Nick. Despite believing himself to be a genuinely brilliant detective, Tim is utterly hopeless, and all of the cases are solved by Nick him despite never getting any credit, with the occasional help or hindrance of supporting characters Chief Inspector Snape of Scotland Yard and his sidekick Boyle. The series compromises three novels: The Falcon's Malteser (1986), Public Enemy Number Two (1987) and South by South East (1991), as well as four novellas: The French Confection, The Blurred Man, I Know What You Did Last Wednesday (all 2003) and The Greek Who Stole Christmas (2008).

The first novel was adapted for the big screen in 1989 under the name Just Ask For Diamond, and South by South East was made into a TV series in 1991 using the same cast.

Tropes used in Diamond Brothers include:

Adults Are Useless: Nick manages to outwit each and every adult he encounters, be it the police, his older brother, or a master criminal.
Bait and Switch Gunshot: Played for Laughs at the climax of The Falcon's Malteser.
Black Dude Dies First: Inverted - Mark Tyler, the only black character in I Know What You Did Last Wednesday, is the final character to die (albeit by a matter of seconds).
Blatant Lies: There are several examples, mostly from Nick trying to cover up Tim's incompetence, but the best example has to be in Public Enemy Number Two, when Tim announces to a group of criminals he and Nick are meant to infiltrate that he is a private detective; as the mood rapidly turns frosty, Nick makes up a spur-of-the-moment tale that he is a private investigator of water, "having to check there is enough H". Incredibly, this works.
Better Manhandle the Murder Weapon: Tim has managed this three times in all, two of which were in the very first book. Nick provides the lampshading.

"Do you think... you don't think the police think I had anything to do with what happened to the dwarf, do you?" Tim asked.
"No," I replied soothingly. "You went up to see him. There was a gunshot. You were found holding a smoking gun. The dwarf was dead. I'm sure the police won't think you're involved."

Clueless Detective: Tim, often taken to extremes; the one time he manages to solve a case, not only is it completely by accident, he's on the run from the law at the time, and he promptly destroys the priceless Ming vase he was meant to be finding by accident. The logo of the series features the words 'Diamond Brothers Detective Agency', with a scratch over the first 't' in 'Detective' making it seem like it actually says 'Defective'.
Comic Book Time: The original books came out in the late 80s and early 90s. The more recent novellas, released over a decade later, have Tim and Nick at the same age but feature modern-day concepts such as the London Eye.
Courtroom Antic: Subverted by Public Enemy Number Two: when Tim is called as a witness to Nick's trial, he does proceed to make a complete mockery of the legal system by saying increasingly stupid things, but he is completely unaware he's doing so and it more or less secures Nick's conviction.
Deadpan Snarker: Nick.
Do Not Call Me "Paul": Tim gets very touchy about people using his real name.
Early Installment Weirdness: The first book refers to Tim as "Herbert" (his real name) throughout, and isn't quite as comedic in tone as any of the others.
Gene Hunt Interrogation Technique: Everything Boyle does.
Hurricane of Puns
Hypercompetent Sidekick: Nick. Whilst Tim faints at the slight of blood and is constantly caught at the scenes of murders holding incriminating evidence (typically the murder weapon), he solves every single case.
Incredibly Lame Pun: Many times, even the titles of the books.
Last-Name Basis: Snape and Boyle; in the former's case it turns out that his first name is Freddie.
Odd Name Out: The first six books' titles are all parodies of crime novels and movies such as The Maltese Falcon and North by Northwest. The last book's title is taken from How the Grinch Stole Christmas.
Parental Abandonment: Inverted in that Nick chose to abandon his parents when they emigrated to Australia.
Piano Drop: Nick escapes from the flat he's imprisoned in by dropping a piano on the villains returning there after finding he's lied to buy himself time.
Plot Based Photograph Obfuscation: The Blurred Man, where the two criminals of the book have been running a charity using the fake persona of "Lenny Smile". When somebody asks for a photo of Lenny Smile, one of the criminals dresses up as him in a deliberately out-of-focus photograph.
Punny Name: In the first book alone there are two Germans called Gott and Himmel, a Femme Fatale called Lauren Bacardi, and a charlady called Betty Charlady.
Reunion Revenge: The plot of I Know What You Did Last Wednesday.
Running Joke: Tim's inability to remember a client's name properly; 'Mr Naples' becomes 'Mr Venice', 'Mr Navels' and 'Mr Nipples' within the course of the first chapter, 'Johnny Powers' becomes 'Johnny Flowers', 'Mr Hammill' becomes 'Mr Camel', 'Mr Hubble', 'Mr Rubble' and 'Mr Rabble', etc. etc.
Schedule Slip: 12 years between books 3 and 4.
Status Quo Is God: No matter how great the reward is for solving a crime, by the beginning of the next book the Brothers are destitute again.
Title Drop: All the titles of the books, from "I Know Who You Killed Last Wednesday" to "The Falcon's Malteser" to even "South by Southeast" get dropped in context.
Too Dumb to Live: Tim
Twist Ending: Once in each of the first three novels; In The Falcon's Malteser, when Nick finds that Lauren has got to the diamonds before him, in Public Enemy Number Two when Nick realises his French teacher is the Big Bad, and in South by South East when Nick realises that Tim's girlfriend is the Big Bad.