Why Achilles' heel but Dickens's novels? Why s' and s's for names ending in s?

Well, according to my Modern Language Association Handbook, only one-syllable proper names ending in "s" should have an apostrophe and another "s" added -- "Keats's poems." Polysyllabic names ending in "s" should take only an apostrophe for the possessive: "Cervantes' books, Hopkins' poems." The MLA would thus prefer "Dickens' books."

However, the Chicago Manual of Style states that while both "Dickens'" and "Dickens's" are correct, the CMS prefers the latter.

So take your pick.

I've also seen official literature with Ellises and the like, and tend to see all three versions used almost at will.

Apostrophes appear to be part of a developing rather than a fixed grammar!

http://www.apostrophe.org.uk

Purdue's OWL (Online Writing Lab) has a handout about apostrophes.

When forming possessives of nouns, they say this:

add •'s• to the singular form of the word (even if it ends in •-s•)
add •'s• to the plural forms that do not end in •-s•
add •'• to the end of plural nouns that end in •-s•
add •'s• to the end of compound words
add •'s• to the last noun to show joint possession of an object

They also say you don't use apostrophes for possessive pronouns (his, her, its, etc) or noun plurals.

Link: https://owl.english.purdue.edu/owl/resource/621/01/