I am currently transcribing a letter from 1845 and having some trouble with the handwriting. This may be a half-baked question but are there any resources available for helping decipher old penmanship, other than referencing other letters of the period? Obviously, I know to look for context, misspellings, etc.
Welcome to the wonderful world of paleography or the study of old handwriting! It basically comes down to blood, sweat and tears, or practice and more practice. There are some guides and tutorials online that cover the essential rookie mistakes and offer exercises, though not many specifically for the 19th century. I do think that you might find the following useful:
National History Museum Palaeography Guide (pdf)
Useful Tips for Reading Handwritten Documents
UK National Archives Palaeography Tutorial 1500-1800
But what you mainly need is a burning desire to read what the text says. I had an easier time deciphering what an ancestral baptismal record written by what appeared to be a drunk priest said once I discovered that it involved the salacious story of an illegitimate birth...
You could also post an image of the document on here and we can all have a go at it.