Following the apostrophe query and answers, 'im indoors found and old book.
Treble H.A. and Vallins G.H. (1936)Oxford University Press London.
Whilst old fashioned and 'wordy', it does have some good general rule of thumb.
'SINGULAR: Singular noun +'s
The only exceptions are classical names like Mars and Venus which make genitive Mars' and Venus'
(a) If the noun ends with an s in the plural: Plural noun + an apostrophe after the final s.
(b) If the noun does not end with an s in the plural: Plural noun +'s. (pg 23)
It is a good working rule NOT to use an apostrophe form of the genitive when the resultant word would be ugly or cacophonous. Thus of a rhinoceros, not rhinoceros's.
'Third person pronouns have no apostrophe: ours, yours, hers, its, theirs.'
Hope that shed's some light, a handy little tome way out of print now but maybe available on e-bay.