The documentation for Google Books states that:

' (apostrophe/single-quote) is treated as a separate word, except when it precedes the letter s, as in Alice's and Bob's

This means that words like can't, doesn't, he'd, and they've are actually composed of three "slots" in the n-grams tables. As a result, they need to be entered into our search form with a space before and after the apostrophe, e.g.:

With 's it is just one word, as mentioned above, e.g.:

it's,   it's ADJ,   that's,   that's ADV,   Mary's,   world's,   the world's NOUN,   *'s troubles

But note that words ending in s' do have a space before the apostrophe, e.g.:

Jesus '  /  Jesus ' life  /  Jesus ' NOUN

However, some of these forms with 's seem to have problems in the original Google Books n-grams data, and they also are composed of three "slots", but this is mainly for the 1890s-1910s, e.g. he ' s, it ' s, world ' s. The strange frequency with cases like Jesus ' NOUN (occurrences early 1800s, late 1900s, but not much in the middle) also makes us wonder what is going on. At any rate, we have kept this data as it was in the original Google Books data.

One last issue: when you go to see the excerpts in Google Books, in most cases the "split" word (e.g. can ' t, or they ' ve) is now joined together again. We're not sure what's going on there, and there's not much that we can do to fix that.