a.co is amazon's in house url shortener....freaking seriously man....

Here you go:

https://www.amazon.com/s/ref=nb_sb_ss_i_3_20?url=search-alia...

Just click the book, then under "Share" click the email button, and you'll see..........

That was my guess too, but this one actually isn't. Amazon uses "?ref" for reference (innocuous), and "?tag" for affiliates.

For quick reference, a.co and amzn.com are official shorteners and considered safe. amzn.to links however are third-party and often used to conceal affiliate links.