SIMONE DE BEAUVOIR LETTERS TO SARTRE, translated and edited by Quintin Hoare (Vintage, $105). THERE is something voyeuristic about reading someone's private correspondence. Like reading a diary which doesn't belong to you. With Simone de Beauvoir's letters to Jean-Paul Sartre, however, the sense that you are invading someone else's private life shamelessly vanishes as you become increasingly absorbed by that life and the way in which it is described. The letters, written from the 1930s to the 1960s, are, above all, love letters written with passion and - refreshingly for a couple who were better known for their intellectual pursuits - a great deal of sentimentality. The couple were separated little during their life-long relationship. The thickness of the book is accounted for by World War II, when the couple were separated because of Sartre's mobilisation and subsequent imprisonment. During this time de Beauvoir wrote to him daily, and with sometimes desperate devotion, for nearly seven months. Notoriously unconventional in their attitude towards relationships, de Beauvoir and Sartre had what they called a ''morganatic union''. While she sometimes refers to him as her ''husband'' and to herself as his ''spouse'', they were never formally married and were always having affairs - Sartre with other women, and Beauvoir with other men and women. Even in her desperate moments of missing Sartre during the Phoney War of 1939/40, she tells him every emotional detail and intrigue of these relationships: her seductions of other women, their demands on her, and her hopes, fears and frustrations concerning them. This relaxed attitude towards each other's sexual exploits has a loophole, however. Despite herself, Beuvoir cannot help being jealous and competitive, not only towards Sartre's affairs - she often declares how much she would like him ''all to myself'' -but also towards female lovers whom she shares with other men. By describing these weaknesses, de Beauvoir's letters are unique in being able to give us the other half of the story: the vulnerable side to this strong, intellectual figure. In telling Sartre about her changing daily moods and loves with such blatant honesty her letters lend an insight which an autobiography - written with the privilege of hindsight and the selectiveness of memory - could not convey so well. The letters are also a diary of de Beauvoir's daily existence; ''the everyday dust of life'', as she calls it. What the edition lacks, however, is helpful autobiographical detail between the chapters - Beauvoir's life between 1941 and 1943, for instance.