Book of Secrets

So i finished up nightfall for the first time today,
however, after the end of the cutscene, my game lagged out
it counted that i completed the mission, i could access hardmode go to DoA ect, however i did not go to the place after the mission and did not get my book of secrets
If this happens, can i repeat the mission to get the book?

Thank you

yup, complete the mission and ye shall be able to get your book.

You don't get the book upon mission completion, you get it after you talk to the keeper of secrets. Talk to him next time you do the mission, and you should get it.

your gonna have to redo the mission then make sure you talk to the keeper of secrets.