At present, recognizing the importance of being proficient in English, it is taught primary grades onwards within the Sri Lankan education system. 'Let's Learn English' is a series designed for primary grades; three, four and five, and lays the foundation for our children's formal English language learning process. Therefore, a critical evaluation of this series could be considered a vital requirement to discover whether the students' language requirements are satisfactorily fulfilled. Drawing on comprehensive qualitative and quantitative data through questionnaires and interviews from a study sample of ESL teachers, instructors and students, this study mainly sought to investigate the extent to which the content of the course book meet the set objectives. And, attempts were made to inspect how far the course books are free from errors. Also, the integration of all four language skills was analyzed to examine whether the communicative needs of students are satisfied. The ultimateobjective of the study was to identify strengths and weaknesses of the course books and recommend improvements to them with the aim of enhancing the quality of existing learning and teaching practices.