Letters were important means of communication. It is estimated that by 1917, the British army on the Western Front was sending about 8 million letters per week,1 so it is not surprising that some of them have been preserved to our days. Here are but a few examples. More can be found in the Europeana 1914-1918 online archive. A related project is crowdsourcing the transcriptions of letters and other texts. To take part, or to simply explore the material, visit https://transcribathon.com/en/
For a more comprehensive introduction to letter writing and postal services during the war, explore the article by Martha Hanna 'War Letters: Communication between Front and Home Front', published as part of the 1914-1918-online International Encyclopedia of the First World War.
1) Roper, Michael: The Secret Battle: Emotional Survival in the Great War, Manchester 2009, p. 50