Not all books will offer the scroll option. If you're not seeing the option, try another book.
Books with fixed formatting won't offer the scroll option, nor will side-loaded books from other sources.
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How to contact Amazon CS:
Use the HELP link at the bottom of any Amazon page, then NEED MORE HELP...then CONTACT US...and CONTACT US again...select CALL or CHAT. The Call option allows you to input your phone number, and they will call you. Don't use email, and don't Google for a phone number, the results are scam numbers.
Thanks teddygrace. I'll do some more digging. Hopefully I can find the setting somehow as I've gotten used to reading my books using this feature.
Wish I'd been able to ask where the option is but, not having a 9th gen, I wouldn't have been able to answer questions about what I was seeing on the screens.
There are separate sections for Support for Fire Tablet (4th gen & later) & for Fire Tablet (8th & 9th gen). The headings for the 2 are similar & the lists of topics seems to be almost identical except some are in a different order.
Tapping on Read on Fire Tablet (& many of the other topics) in the 8th & 9th gen list goes to a page that has "Support for Fire Tablets Released in 2014 or Later" at the top & presumably means the info is the same for all gens, 2014 thru 2019, OR that the 8th & 9th gen Support pages haven't caught up.
And, that page is obviously not the place to go if you need support for reading on a Fire!