When she opened the book I thought she gasped at the photo of Albert's mother. I assumed that she recognized the woman in the photo and thought she found a scandal about some out of wedlock duchess giving birth as Albert said his mother was supposedly of rank. So how did she find out Albert was a woman from the book, was it a diary?
Eagles may soar but weasels never get sucked into jet engines
No, Mrs. Baker discovered something else, but it's a SPOILER.
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It's never implied in the film that Mrs. Baker discovers Albert's true identity after his death. She is looking through the small ledger in which he recorded his earnings, then eventually finds his stash of money hidden under the floorboard.
Challah if you hear me
You're correct - the gasp is because she realizes this little diary is a ledger for saved money which is almost certainly in the room and she closes the door. We don't see her find it but obviously she has given she now suddenly has money to fix up her establishment when she was on the brink of ruin before.
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