The second way 's' is pronounced at the end of a word is 'sss'. It's pronounced this way after voiceless consonants 'puh', 'tuh', 'kuh', 'ff' and 'th'.
The second syllable, unstressed, begins with the F consonant sound, ff, ff, so the bottom lip will come up and make contact with the bottom of the top front teeth, ff, ff, -ffee, -ffee, -ffee.
You might be on holiday, or on a long train journey, ff it is a good book, you might say, It's so good that I can not put it down. But not all stories belong to this class.