In this work, I present a linguistic investigation of the language of Swedish textbooks in the natural sciences, i.e., biology, physics and chemistry. The textbooks, which are used in secondary and upper secondary school, are examined with respect to traditional readability measures, e.g., LIX, OVIX and nominal ratio. I also extract typical linguistic features of the texts, typicality being determined using a proposed quantitative method, labelled the index principle. This empirical, corpus-based method relies on automatic linguistic annotations produced by language technology tools to calculate what I call index lists, rank-ordered lists of characteristic linguistic features of specific text corpora as compared to reference texts. I produce index lists for typical vocabulary, noun phrase structures and syntactic structures, extracted from a 5.2 million word textbook corpus, compiled as a part of the work presented. As well as being frequent and well dispersed, the linguistic variables selected for the index lists are also characteristic of the text type in question, as is evident when they are compared to a reference corpus, comprising textbooks in the social sciences and mathematics, as well as narrative and academic (university-level) texts. The results show that textbooks in natural science contain a lot of content-specific, technical vocabulary. This characteristic not only distinguishes natural scientific language from everyday language, but also from social scientific language, which on the lexical level has more in common with narrative texts. On the other hand, the textbook language as a whole is structurally distinguishable from narrative texts, as clearly seen, e.g., in its noun phrase complexity. In the transition between secondary and upper secondary school, the scores of almost every readability measure go up, indicating an increase in linguistic demands on the readers. In the upper secondary textbooks the words are longer, the vocabulary more varied, the noun phrases longer and more elaborate, and the most typical syntactic structures more complex. Notably, the linguistic development between the form levels is more marked in the natural-science textbooks, compared to social sciences and mathematics. Nevertheless, the textbook language overall shows a relatively low complexity in comparison to academic language.