Perhaps best read as a series of essays, as the only common thread is, unsurprisingly, salt. 
While interesting, it lacks any forward momentum, any clear overarching point other than "everyone needs salt".
As others have said, "a world history" is a bold claim for a book that spends the first two chapters briefly covering China and Africa, before seeming to completely pivot to Europe and it's descendants.
The fact that so many cultures around the world discovered largely the same best methods to collect salt is an interesting factoid to tell at a party, not as interesting to read chapter after chapter. The recipes similarly lose their novelty once they stretch past the single page mark.
Someday I hope to return to this book and finish it, but for now I feel no strong urge to continue reading.