For me (tho I have several) it is a particular edition of Hans Anderson Fairy Tales which I owned as a child. I don't know what happened to it but I still search the junk shops for that exact edition. I can remember the musty smell, the colour of the cover, the loose pages and the memory of reading it under the bedcovers by torchlight. Another would be Maisie Middleton which I read to my daughter every night when she was little, for probably more than a year ..I can still recite it from memory.
I would be great if you would send me a photo or scan of your book cover (tho don't worry if not)and a line about where or when you acquired it and what it means to you.
As well as being incredibly helpful to my research I would like to include them in a book... only with permission of course.
..thanks in anticipation