A book that makes you happy.
A real feel-good book for me is "The Complete Sherlock Holmes" by Sir Arthur Conan Doyle. It's the sort of book I take with me into my favourite armchair, curled up into it by the fire with a good glass of wine and a plate of tidbits, so that I can lose myself in Victorian England quite thoroughly. Up up and away! :)
Also runners are...
"A Tale of Two Cities" by Charles Dickens
"Sense and Sensibility" by Jane Austen
"Master and Commander" by Patrick O'Brian
"Murder on the Orient Express" by Agatha Christie