Yes please! Being a novice I definitely would like to know about what to look for, what makes them more valuable, and how to avoid fakes.
I am no expert in any way, just a fellow collector. So I can give you my perspective and trends I have seen over the years.
In general, I have found that the content of a letter effects value quite a bit. If a letter describes a battle or historic event, it will be more valuable than a letter that has a more mundane topic. Fame also effects value. I can't afford letters from famous generals, but they would be more valuable. In the same sense, a letter from a famous unit, like the "Stonewall brigade" or the "Iron brigade" would have more value than a lesser known unit, all else being the same. Because they are rarer, Confederate soldier letters tend to have more value than Union soldier letters, again, all things being equal. So, you did well getting a Confederate letter. Again, content seems to be most important. A Union letter from a soldier in an unknown regiment that has a beautifully written vivid description of the battle of Antietam or Gettysburg would be more valuable than a Confederate Stonewall brigade soldier letter simply lamenting camp life. And, how legible and complete letters are does matter some. I like when a letter comes with it's envelope, but that isn't the most important thing for some.
We are all different though, so much depends on what you like. I don't know if you are collecting just any letters you find interesting or whether you have a particular area of focus or interest that you are interested in. For instance I had ancestors that fought and I like to find letters that might relate to them somehow. As I said, I inherited some war dated family letters, so I treasure them. And they are not even "soldier" letters. I had ancestors in the 87th PA Infantry and have never found any letters from that unit, however, I have found interesting letters from soldiers in regiments that were in the same Brigade as the 87th. That is neat to me because they could have camped or fought close to one another. I also look for letters related to battles my ancestors were in. So, I have honed down my collecting over the years. You may want to collect whatever interests you or you may want to have a personal focus. And, to be honest, sometimes sales just happen. A fellow found a collection of items when he moved into a house and called me. We worked out a deal for the whole group. That has nothing to do with my ancestors, but they are a treasure too!
One letter I was willing to pay a bit more for was from a Confederate soldier of the 18th VA Infantry. I have two ancestors, brothers, on my mother's side, that fought in the 2nd VA Infantry and one of them was Killed on August 29, 1862 at the 2nd battle of Manassas. The letter I found described the 18th VA's participation in the 2nd battle of Manassas in detail and even mentions how the 29th of August was one of the hardest fights the soldier had been in. Adding to the value of the letter for me is the fact that the writer was in Richard Garnett's Brigade of George Pickett's Division and he ended up being one of the thousands of soldiers that lost their lives during that most famous charge on July 3rd at Gettysburg. So, there was a personal connection to the 29th of August, 1862 and an awful lot of meaningful history that came with that letter.
I am like you though, I like all of my letters because they humanize the history - make is personal and help me better understand that these were people not so different from ourselves. One letter I have isn't very valuable because it is a sister writing to her soldier brother of the 14th New Hampshire in a hospital in Washington DC. She talks about what was happening in their home town and how her son went fishing where the soldier used to fish with him. Nothing very interesting except for peoples daily lives. Yet, one thing I find interesting about it is the date, July 5, 1863. She does mention that she could hear cannons fired for Independence Day in the nearby town, but she doesn't mention the big three day battle that just happened in Pennsylvania. It is not like us with CNN. Who knows when people in her town would have found out about Gettysburg? It was a different time!
Enjoy you collecting. I do. I can't say much about authenticity except buy from reputable sellers or know the history or provenance of the items if you can. When I bought the grouping I went to the farm where the family was from and there was even a picture of the soldier sitting in front of it, so that is about as good as it gets.