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
An Inquiry Concerning Al-Mahdi
The Promised Savior: An inquiry into the imamate of Imam Mahdi (as) from the viewpoint of Muslim thinkers
The Occultation of the Twelfth Imam (A 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):
Hope this is helpful inshaAllah.