"De spin leest een boek."
Translation:The spider is reading a book.
It can always mean either of two tenses in English. It just depends on which English tense is better suited in each case.
In the third person leest can mean "(he/she/it) reads" or "(he/she/it) is reading".
Or in the second person it can mean "(you) read" or "(you) are reading".
We have these two different tenses in English, which can mean two different things. However in many other languages only one tense is used to express both of these ideas. (For example German and Spanish are also like this).
[edit- oops, yes in Spanish progressive present also exists - thanks JCMH!]
You are right in the case of German (only where people until 50 to hundred years ago spoke a language related to English or Dutch you can hear the continuous form), but i am quite sure that in Dutch you can express that s.o. is just doing it in different ways. Here a " de spin sit een boek te leezen' would probably do.