If you are responsible for the marketing of your business, a rudimentary understanding of SEO is essential even if you are planning to outsource. Here’s some resources to help.
If you are only going to read one document (approx 1 hour reading time) then read Google’s own Starter Guide. Of course it’ll teach you to optimise in the way Google wants you to optimise which is no bad thing, especially if you are only going to invest an hour.
If you’ve got time for a book, then “The Art of SEO” does the job nicely.