Selecting reading materials that are appropriate for adults with low literacy skills remains a central challenge in Adult Learning contexts. This challenge becomes particularly acute when the unit of analysis is not a short passage but a full book or long-form text, where internal heterogeneity in lexical, syntactic, and discourse-level properties make global readability estimation non-trivial. In practice, librarians, educators, and publishers often need to make decisions about the suitability of books for specific learner populations without access to complete texts, relying instead on partial excerpts or limited samples, and personal intuition.