Hi! I wonder if there's a good book with an impostor/hidden identity theme. Like a girl pretending to be a boy or a spy assuming a false persona (like "It's Not Easy to Be a Man After Travelling to the Future" for example). If it has a lot of chapters (500+) that's even better. Thanks!

Guiltythree Yes. I recommend my novel, 'The Elusive Book Keeper'.
https://dynamic.webnovel.com/book/17707119706580305?utm_source=writerShare&utm_campaign=4313215511
The Mc is an elite student treasured by the nation and at the same time, he grows as an intelligence expert that sells information to the highest bidder.
I believe you will enjoy it.

https://www.webnovel.com/comic/love-x_14710505205373801/002_39534339226128288
YES MY READ is Love- X right now

Guiltythree Uhh kinda self promo but my story is about secret identities ^- It's kinda short right now, but I've been trying to update every week ^-

Transmigrated as the Emperor's Nanny

https://www.webnovel.com/book/transmigrated-as-the-emperor's-nanny_21091578206415405

Former master assassin, Code X, wakes up as a 74 year old woman dressed in ancient clothing. Instead of a toned body and youthful features, she now finds herself with the body of a grandma, complete with a walking stick, white hair, and bad eyes!

How did she end up with this situation? Simple — this was karma for all the blood on her hands. To atone for her past sins, she now has to save lives. The more lives she saves, the younger she becomes! But this is not as simple as it looks, not when she finds herself reincarnated in a warring kingdom plagued by death and crime.

So when she saves the life of a young boy, she does not expect his next words: “Thank you, beautiful elder sister.”

When she touches him, she momentarily regains her youth? But wait, this poor-looking boy is the Crown Prince? And she is now his nanny? What will she have to do to escape this weird situation?