Assalamu aleykum,

Thanks for your question.

There are many interesting and well-written books about Imam Mahdi (aj).

I recommend the following ones:

Al-Imam al-Mahdi, The Just Leader of Humanity

https://www.al-islam.org/al-imam-al-mahdi-just-leader-humanity-ayatullah...

An Inquiry Concerning Al-Mahdi

https://www.al-islam.org/an-inquiry-concerning-al-mahdi-sayyid-muhammad-...

The Promised Savior: An inquiry into the imamate of Imam Mahdi (as) from the viewpoint of Muslim thinkers

https://www.al-islam.org/promised-savior-inquiry-imamate-mahdi-viewpoint...

The Occultation of the Twelfth Imam (A Historical Background)
https://www.al-islam.org/occultation-twelfth-imam-historical-background-...

That is just one small portion of texts concerning Imam Mahdi (aj) that can be found at Al-Islam.org, there is a whole section dedicated to him (aj):

https://www.al-islam.org/library/imam-al-mahdi

Hope this is helpful inshaAllah.