I am an author, and I have books stored with me. Is it permissible to pay my zakat in the form of books containing beneficial knowledge for poor students, such as chemistry and mathematics, or religious books that spread Islamic sciences?

Zakat on wealth must be paid in cash from the wealth itself. Therefore, you should pay your zakat in monetary form so that the poor can purchase what they need. And Allah Knows Best.

Is it permissible to listen to chants praising the Prophet if accompanied by musical instruments?

If singing, or chanting in praise of the Prophet was accompanied by a forbidden musical instrument, then it is forbidden.

What is the Du`a (supplication) of Istikhara (guidance prayer)?

O Allah, I consult You as You are All-Knowing and I appeal to You to give me power as You are Omnipotent, I ask You for Your great favor, for You have power and I do not, and You know all of the hidden matters. O Allah! If you know that this matter (then he should mention it) is good for me in my religion, my livelihood, and for my life in the Hereafter, or he said: "for my present and future life" then make it (easy) for me. And if you know that this matter is not good for me in my religion, my livelihood and my life in the Hereafter, or he said: "for my present and future life" then keep it away from me and take me away from it and choose what is good for me wherever it is and please me with it."

I prayed all the obligatory prayers in congregation, but after finishing the `Isha prayer, I remembered that I had forgotten to pray Dhuhr. What should I do, and is my prayer invalid?