According to the book I read, "Love is the pearl of great worth" is grammatically wrong. Why is it? (It says "the" is [sic].)
It is not ungrammatical. The choice between the definite and the indefinite article depends on the context, and on what exactly the writer is trying to say.
The origin of the expression is biblical. It occurs in Matthew 13:46 of the King James Bible as
Who, when he had found one pearl of great price, went and sold all that he had, and bought it.
Google returns 71,300,000 hits for ‘the pearl of great worth’ and 87,700.000 for ‘a pearl of great worth’. The figures for ‘the pearl of great prce’ and ‘a pearl of great price’ are roughly the same at more than 38,000,000.
This nGram graph shows a narrowing of the gap between both ‘the pearl of great worth’ and ‘a pearl of great worth’. This one shows that ‘a pearl of great worth’ is comparatively rare, and doesn’t show ‘the pearl of great worth’ at all.
I would probably reword it as "Love is a pearl of great worth."
Using "the" implies that there can only be one "pearl of great worth."
The book editor is taking issue with the content indicating with sic that there is indeed more than one pearl of great worth, not that the sentence is incorrect grammatically. The literal meaning of sic is thus, and the editor is reiterating that this is the original version. If I were writing on the well-known poem "On First Looking into Chapman's Homer", which ends
Then felt I like some watcher of the skiesI have added the sic because it was the explorer Bilbao, not Cortez, who discovered the Pacific Ocean (for Europeans, Asians and Native Americans having discovered it long ago).
When a new planet swims into his ken;
Or like stout Cortez [sic] when with eagle eyes
He star'd at the Pacific — and all his men Look'd at each other with a wild surmise —
Silent, upon a peak in Darien.
In my amateur reading of the original usage in the Biblical verse, the implication is that the Kingdom of Heaven is indeed a uniquely valuable pearl, of the greatest price, which may have influenced the author.
By adding [sic] I don't think the editor necessarily implies any criticism. As is normal in such cases, he's simply making sure the reader doesn't assume there might have been a transcription error.
In fact, I'd say almost certainly the writer deliberately chose to use the definite article "the", even though most readers would be expecting the indefinite article "a" in that context.
The writer is making an oblique reference to the "proverbial pearl". He could have various reasons for doing this, but one possibility is he doesn't want the reader to think he's just passing off a tired old cliché as his own "creative prose". And the editor is just making sure the reader realises that.
EDIT: OP hasn't provided us with the original source, but I found half-a-dozen candidates in Google Books (the earliest seems to be 1928). It's worth flagging up the quote marks in this one (from 1931), where it's love is the "pearl of great price". Not to indicate a special usage or an unusual phrase just coined by the writer, but to show it's a known (to many) "quote". Notice how the definite article isn't included within the quote marks. I'm quite happy to accept that OP's writer felt his more clued-up readers would recognise the sense without that distracting punctuation.