This is the second of the Tommy & Tuppence Beresford books - some time has passed since the events of The Secret Adversary, and they are now quite the staid married folks. Or not.
Partners in Crime is a series of linked short stories - the British government has need of the Beresfords again, to take over a detective agency that has been used in some sort of espionage ring. While Tommy is pretending to be Mr. Blount, with Tuppence as his trusty sidekick and secretary, they are hired to solve some crimes in addition to the spy ring they are trying to bust. There's a stolen pearl, a poisoning, and some counterfeiters, as well as providing some assistance to a lovelorn young man who is trying to break his beloved's alibi for a bet.
Both Tommy and Tuppence are fans of the detective story, so they try out the personas of several of the best known detectives of the time, many of which I've only vaguely heard of, but who include the master of ratiocination, Sherlock Holmes, and he of the moustaches and leetle grey cells, Hercule Poirot. This is delightful.
In fact, the whole book is delightful. The mysteries are largely forgettable, but the banter between Tommy & Tuppence is engaging, and the two of them are adorable, bubbly, cheerful and warm.