The Prussian influence to unite Germany into a single empire began in the 18th century under the leadership of Frederick the Great (1749-1786). In 1818, Prussia was the originator of the Zoliverein or Customs Union which, by 1844, included all of northern Germany. This led initially to the ease of transfer of goods but inevitably towards postal union which was finally agreed with Austria and most German states in 1850.
Stamps were issued by 16 different States, the first stamp being the Bavaria 1849 1k black. In 1868 the stamps of the North German Confederation replaced those of Bremen, Brunswick, Hamburg, Lubeck, Mecklenburg, Oldenburg, Prussia & Saxony.
Although Bavaria & Wurttemberg continued to issues stamps into the 1920s stamps for the unified German Empire commenced on 1 January 1872.
We recommend the excellent stamp listings in the Stanley Gibbons Part 7 “Germany” catalogue, also in the Michel “Deutschland-Spezial” Volume 1.