This study investigated the relative effectiveness of books on reading achievement scores when earned by under-achieving readers in a student tutoring program. Fifty-three fifth grade students who were in below grade level classroom reading groups and who scored from six months to three years below grade level on at least one of two reading achievement tests, were assigned to three treatment groups and one control group. The Wide Range Achievement Test and the Slosson Oral Reading Test were administered before and after the treatment phase. Data, in the form of the number of lines read correctly and number of errors made during each of the tutoring sessions, were collected. One group received knowledge of results only. A second group received knowledge of results and gift certificates redeemable at a local toy store. The results showed that the subjects reinforced with certificates for books showed significantly greater gains in reading achievement scores than did subjects who obtained other reinforcers. (Author/TS)