Oh this is such a good question and as I read through the choices I spotted many I might have put at the top and would be sad to lose.
However, my granny gave me a replica of the Kilmarnock edition of Robert Burns's poems, reproduced by the Scottish Daily Express in 1938. It includes works published in Rabbie's lifetime and later works, some of which were written by others and added to in the same style by Mr Burns. My reasons are 1) It would take ages to work through the Lallans Scots, even with the copious explanations; 2) those notes are stories in themselves; 3) all human life is here in these pages. I do realise he was not the best hero for us to have in his personal life, but if only one literary treasure is allowed, this would be mine.