Possessive case for words in singular ending on "ss"

[Caprilla]

Hello, everyone!

I've got a question about using 's with words like "boss", "mistress" etc. when putting them in singular. I.e.: My boss's car is damaged.
I'm aware that the rule says we should but my son's students book has the following examples (picture attached): Tess', Ross'. Should I take it as it refers only to names?

 

[5jj]

Can you give us the title and the author of this book, please?

The natural and correct form of these words is mistress's, boss's, Tess's.

 

[Caprilla]

Thanks for your reply! The authors are J.A. Komarova, I.V. Larionova, J. Perrett. The series is called "Brilliant".

 

[5jj]

Thanks.

I know neither the authors nor the series, but I do know that they are mistaken here.

 

[Caprilla]

Thanks.

I know neither the authors nor the series, but I do know that they are mistaken here.

No wonder you don't. These books are written for Russian primary schools.