We investigated how SES, grade level, and book reading experiences are related to children's writing self-efficacy as well as to their knowledge of "good writing" and "writing difficulties." The sample included 199 middle-high (HSES) and low (LSES) SES children (63 second graders, 67 fourth graders, and 69 sixth graders). Gender and SES (low and high) were approximately equally represented. Children were recruited from two elementary schools, one in a low SES neighborhood and the other in a middle-high SES neighborhood. Writing knowledge was elicited by two open-ended questions and self-efficacy by a 10-item questionnaire. Children's book reading experiences were measured using the Title Recognition Test (TRT). Results showed that, across both SES groups, children's grade level was the most important predictor of their knowledge of "good writing" and "writing difficulties" and also of their writing self-efficacy, followed by the children's book reading experiences. HSES school children gained more writing knowledge as a function of grade level than did the LSES school children. An important and surprising finding was that children's knowledge of "good writing" and "writing difficulties" was explained by their book reading experiences only in the LSES group and not in the HSES group. The implications of these findings for theory and practice are discussed.