The Apple Books app is the most integrated way to buy audiobooks without a monthly commitment.
: Tap the Buy button (or the listed price) to purchase using the payment method linked to your Apple ID. Listen : Your purchase appears instantly in your Library . 2. Using Audible (Subscription-Based)
Buying audiobooks on an iPhone is straightforward, but the process depends on whether you prefer a one-time purchase or a subscription service. 1. Using the Apple Books App (No Subscription)
Audible is popular for its massive catalog and monthly credit system. Buy books and audiobooks on iPhone - Apple Support