English textbooks play a pivotal role as the primary sources of English input for Indonesian students. Hence, this textbook analysis study aimed to investigate the conformity of English textbooks' reading materials to the K-13's genre-based materials. Also. It aimed to evaluate the quality of reading material presentation in the English textbooks in support of English acquisition as informed by the comprehensible input theory. The analysis framework was designed resting upon the systemic functional linguistic theory. The analysis incorporated five English textbooks designed for senior high school students in Indonesia as the analysis objects. Thirty reading passages were selected purposively from the five textbooks as the representatives of analysis objects. The study's trustworthiness was maintained seriously. The findings demonstrated that the English textbooks' reading materials, in general, were aligned with those of the K-13. Nevertheless, the textbooks presented few text types not recommended by the K-13 manual, and one compulsory text type was absent. This study endorsed the comprehensible input theory to provide a positive and constructive criticism to English textbook developers in order to make proper revisions of the materials supporting students' ease of English acquisition and teachers' ease of material delivery. Implications, limitations, and suggestions for future's research are given in the end.