Novel by Irish writer Flann O'Brien (real name Brian O'Nolan, also known as Myles na Gopaleen). The manuscript was rejected by his publishers, and O'Brien was so abashed that he denied its existence for the rest of his life, although he did incorporate some of its elements into later work, such as The Dalkey Archive. The novel was published posthumously, and is now widely considered, along with At Swim-Two-Birds, to be his best work. Certainly, it is his most consistent and coherent, despite its extensive flights of fancy.
It would be wrong to give too much of the plot away, as there is a nice twist at the end (which you may or may not see coming), but essentially it revolves around a rural police station inhabited by an eccentric bunch of officers. The narrator visits the station after taking part in a burglary which has gone disastrously wrong. One of the officers he encounters can only conceive of bicycle-related crime, while the other is occupied with creating mind boggling inventions, including a series of boxes, each of which contains a smaller box in a seemingly endless series, until the boxes become so small it hurts to think about them. As well as the aforenamed preoccupations, the policemen appear to have responsibility for a mechanical chamber of some sort, and have to check various guages regularly, and make the necessary mysterious adjustments.
The plot itself is fairly inconsequential, although not nearly so rudimentary as in O'Brien's other novels, such as The Dalkey Archive or The Poor Mouth. Much of the novel's genius is contained in the footnotes, which describe events in the career of the fictional scientist DeSelby, whose experiments included looking back through time with the aid of a complex series of mirrors. Other elaborate flights of fancy include the famous mollycule theory which envisages that a man riding a bicycle over a prolonged period will inevitably accrete bicycle molecules and thus eventually become more bicycle than man.