To add to the previous writeup, Major Boothroyd
introduces the PPK to Bond with the following dialogue:
"Walther PPK. 7.65 mil, with a delivery like a brick through a plate glass window. Takes a Brausch silencer with very little reduction in muzzle velocity. The American CIA swear by them."
In the five Bond novels before Dr No, Bond was issued with a Beretta 21, a .25 pistol. Ian Fleming received a letter from a fan - a Major Boothroyd (retired) - arguing that Bond would be more likely to use a Walther PPK, and Fleming agreed. Dr No was chosen as the first Bond film, and thus we do not see him carry another issue gun, although the movie has a couple of mistakes (at one point Bond's PPK is substituted with a silenced Browning 1922). Fleming then borrowed the name Boothroyd for a character better known as Q.
Bond's PPK fires the common 7.65x17mm round, also known as the 7.65 Browning, or .32, although the PPK was available in a variety of other calibres, most commonly .380. The 7.65 round is less powerful than the .380 - which is why Bond tended to shoot his opponents several times - although it is smaller, lighter, and has less recoil, important considerations for a secret agent. For a crack shot such as Bond, these outweight the lack of power. In the John Gardner novels, Bond changed to a 9mm ASP in the early 1980s (during which time he also drove a turbocharged SAAB), whilst in the films Bond switched to a Walther P99 in 'The World is Not Enough'.
The 'American CIA' did in fact swear by the PPK, for a time - and so did the CIA's WW2 equivalent, the OSS, not least because being a German firearm the PPK could be stored and used in Occupied Europe without seeming out of place. In 1968 Walter released the similar-looking, but smaller Walther TPH, which also became a CIA favourite (this time in .22). Many other companies have released PPK-style pistols, such as the Mauser HSc and the Cz50. Russia's widespread Makarov pistol owes a debt to the PPK, as do various Hungarian FEG designs. The PPK remains a design classic, being both functional and attractive.
Brausch appear to have been a real company - pronounced 'Brow-sch' - although the internet appears to be devoid of any further information.
The PPK has a further claim to fame; Adolf Hitler owned two, one in .380 and the other in 7.62mm, the latter of which he used to end his life on April 30, 1945.