Thru the end of the month, you can double the value of your credits at Audible's Buy One Get One Free Classics sale. If you are short of credits, you can get three at a discount on the Extra Credits page (if you don't have a membership that uses credits or you have more than 3 on your account, you'll get an error page).
Amazon and Audible have been changing the free companion editions that are available (5 that were free the first day are already changed and now link with other, often non-free editions, such as Treasure Island and Little Women). There are a few new ones, though, linked to free or low-cost Kindle editions, along with a few low-cost companion editions that are linked to free books. On interesting looking one is the self-published Haunted on Bourbon Street (Main/UK), the first title in the Jade Calhoun series by Deanna Chase, with a $1.99 companion audiobook, narrated by Traci Odom.
Book Description
Jade loves her new apartment--until a ghost joins her in the shower.
When empath Jade Calhoun moves into an apartment above a strip bar on Bourbon Street, she expects life to get interesting. What she doesn't count on is making friends with an exotic dancer, attracting a powerful spirit, and developing feelings for Kane, her sexy landlord.
Being an empath has never been easy on Jade's relationships. It's no wonder she keeps her gift a secret. But when the ghost moves from spooking Jade to terrorizing Pyper, the dancer, it's up to Jade to use her unique ability to save her. Except she'll need Kane's help--and he's betrayed her with a secret of his own--to do it. Can she find a way to trust him and herself before Pyper is lost?
Here are few others I've turned up, including some large publisher classic editions that are now also linked to the free audiobooks.
Keep in mind, that except for the titles "sold by" one of the big publishers, those adding low-cost editions via KDP can lie about the publisher - always check the sample to make sure you are really getting what you are paying for (a Penguin Classic, for example); if it is just a scanned edition, you can return it for a refund anytime in the first seven days after purchase.