I watched the 3 hour movie version first and I didn't understand half of what was going on due to there being a TON of characters and Finnish names that I have trouble remembering.
I also didn't know much about the Fin war against Russia so did some research on it.
But, I read the novel by Vaino Linna. Helped me to understand what was going on in here. I loved the book, was a blast to read. I had to write down all the character names to remember, but it helped a lot.
This version follows the book very closely too, down to exact quotes. The book however doesn't show what each character did on leave, so this added a bit there.
I have not seen the 1955 or 1980's version of The Unknown Soldier as I have had great difficulty in finding them... but I was told they are not as faithful to the book.