A very useful tool that latin-derived languages use is making some questions about the sentence, in "ils nous lisent un livre", which is the subject? -ils/they(male) what is the action? -lisent/read, then, who is reading? -ils lisent/they read, what are they reading? -un livre/a book, so ils lisent un livre/they are reading a book, but, adding the "nous" between the subject and the verb, gives a direction to the action, they are reading the book to someone, in this case, us.
They are reading/read us us a book.
The third person plural ends in ent which in this case makes you pronounce the letter before the ent (but not the nt). It is subtle but the only way to get this sentence right is to hear the voiced s (as a z sound). The t in "lit" is not pronounced. This happens in other verbs also but many verbs will sound the same in the third person singular and plural forms if the third person singular already has the ending pronounced such as parle and parlent.
This is a tricky example, because you need to listen for the 's' sound at the end of lisent. At first I thought the sentence was a silly construction (how could multiple people read a single book?), but then I realised that Duolingo probably did this on purpose to make us notice cases like this.
Kudos to Duolingo on the discussion feature. This example and the discussion made me realise the difference in pronunciation between liz and lisent, and that will make it stick.
I recommend opening another tab and bookmark this site: http://www.conjugation-fr.com/conjugate.php?verb=lire