Source, see also this, for instance.In the case of a book belonging to Cassius, we will use the 'Chaucer' rule to place the apostrophe.
Pretend Chaucer Cassiuss book Cassiuses book
Modern correct form Cassius's book
Just apply the rule and the apostrophe will end up in the correct place. This clearly demonstrates that Cassius is singular, i.e. there is just one Cassius we are talking about, and he possesses the book.
This, however, allows for both.
The way I was taught, the genitive case in english should always be formed with the addition of an "'s" for singulars, however going without seems ok today with words ending with an "s", as it's now become more a matter of phonetics.
Initially (when the genitive form first was shaped in writing) it was a matter of spelling following phonetics as well, but for a while it was more about adhering strictly to the "correct" written form, which is where what I was taught stems from, I guess.
From what I understand, as an aside, the addition of an apostrophe was accidental, I don't think it's clear exactly why it happened. We never added it in swedish.