Historical letters & diaries - Dates in the 19th century

Hello,

I'm looking to find historical salutation of written correspondences in the UK during the following periods: around 1800 (late 18th and early 19th century) and from 1900 to 1918 (late Victorian era and WWI era). Research gravitates around general rules and the specific situations described below.

Advice and/or references are welcome.

Samples are pretty easy to find but I'm looking for expert advice on the use of salutations (both at the beginning and end of letter) depending on the nature of the relationship between the senders. What subtleties would exist between two close or distant family members, between a mother/father and her daughter/son, siblings, a contractor and their employer, lovers, close male/female friends, acquaintances, neighbors, etc.

I'm also interested in the impact of their social status: what if (specifically) a chatelaine wrote to a contractor in 1795, a contractor wrote to a lady in 1800, a countess wrote to a lady in 1865, a contractor wrote to a chatelaine in 1910, all in the perspective of a business relationship?

Looking forward to your answers. As stated above, pointers to the proper resources to use would also be super welcome. Thanks in advance!