Surprised by Joy: The Shape of My Early LifeSurprised by Joy: The Shape of My Early Life is a partial autobiography describing Lewis' conversion to Christianity. The book overall contains less detail concerning specific events than typical autobiographies. This is because his purpose in writing wasn't primarily historical. His aim was to identify & describe the events surrounding his accidental discovery of & consequent search for the phenomenon he labelled "Joy". This word was the best translation he could make of the German idea of Sehnsucht, longing. That isn't to say the book is devoid of information about his life. He recounts his early years with a measure of amusement sometimes mixed with pain.
However, while he does describe his life, the principal theme of the book is Joy as he defined it. This Joy was a longing so intense for something so good & so high up it couldn't be explained with words. He's struck with "stabs of joy" throughout life. He finally finds what it's for at the end. He writes about his experiences at Malvern College in 1913, aged 15. Though he described the school as "a very furnace of impure loves" he defended the practice as being "the only chink left thru which something spontaneous & uncalculating could creep in." The book's last two chapters cover the end of his search as he moves from atheism to theism & then from theism to Christianity. He ultimately discovers the true nature & purpose of Joy & its place in his own life.
The book isn't connected with his unexpected marriage in later life to Joy Gresham. The marriage occurred long after the period described, though not long after the book was published. His friends were quick to notice the coincidence, remarking he'd really been "Surprised by Joy". "Surprised by Joy" is also an allusion to Wordsworth's poem, "Surprised by Joy-Impatient As The Wind", relating an incident when Wordsworth forgot the death of his beloved daughter.