Hi Tremalnaik,
Welcome to the Apple (user-to-user) Community.
Re: Books App on iPad:
Apple support have articles online with tips that may help:
Read books in the Books app on iPad – Apple Support
Eg: says: Change the text and page appearance
To Start: Tap the Menu button ie: the page icon with two horizontal bars and three dots underneath",
at the bottom of the page,
Next: tap Themes & Settings, then do any of the following:
✽. Change the font size: Tap the large A to increase the font size or tap the small A to decrease it.
✽ Change the font: Tap Options, tap Font, select a font option such as Georgia or Palatino, then tap Done.
✽ Turn on vertical scrolling: Tap the icon "page with scroll bar on left" to scroll continuously through a book.
✽ For PDFs, tap the icon with the small and large "A", at the top of the screen then turn on Vertical Scrolling.
✽ Adjust the display brightness: Tap the black and white circular icon, then choose an option.
✽ Change the page background color: Tap a page theme such as Quiet or Bold.
✽ Make the font bold: Tap Options, then turn on Bold Text (green is on).
✽ Customize spacing and justification: Tap Options, turn on Customize (green is on), then drag the sliders left or right to adjust line spacing, character spacing, and word spacing. You can also turn Full Justification and Allow Multiple Columns off or on (green is on).
To Finish: Tap "Done" to apply your changes.
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Re: "app control became small and many functions like page turning is gone.
I don't understand the craze of making the controls appear and disappear"
If you wish, you can send Feedback on the Books app to those behind the scenes who can make changes:
They will likely not reply, yet if others are also experiencing difficulties we may see changes in an update.
Here's a link to the web-form for Feedback:
Feedback - Books for iOS and iPadOS - Apple
All the best :-)