Not talking about comics, rather on books (like actual novels containing chapters consisting of only words). They say that both French and German are “closest” to English but still different languages, however even if an English speaker were going to learn either language, does it mean can they understand written text from books or is that hard even with cognates?

It depends on the genre of the reading material, also affects difficulty regarding vocabulary. Such as: thriller (crime): has terminology, uses expressions and bundled in sentences that are beyond A2 level proficiency while YA (teen) may be written within a beginner lens. This is not the same as reading a sign in French as a English speaker as books have blocks of text.

Even in the reverse: what is the recommended level (from A1-C1+) for a non native who is learning English as a second language to read an entire book (like those written by Lee Child, Agatha Christie) fluently with understanding not only the contents written but infer on the novels concepts and relay reading comprehension akin to a native speaker?