"sheets" in a book are generally known as pages, the only exception is perhaps a book of music.
lembar = sheet (of paper)
halaman = page
A sheet of paper has two sides.
A sheet of paper in an analog book has two pages.
You can try this at home:
Tear out one sheet out of your book, and see how many pages you're missing.
I have never heard people talk about a "sheet of a book" though. It's undefined and therefore cannot be known nor distinguished from the non missing sheets.
That's not how it works in English.