A lot of Sunni scholars have written books regarding Imam Mahdi (hgr).

For example, the first one who wrote an independent book about Imam Mahdi (hgr)

was “Ibād ibn Ya’qūb Rawajati”.

His book is “Akhbār Al-Mahdi”.

He lived in Imam Askari (pbuh)’s lifetime,

and passed away in 250 AH.

Some other examples [of scholars] are as following_ that their death date should be noticed too:

۱-The book “Ahādith Al-Mahdi wa Akhbār Al-Mahdi” written by Abūbakr ibn Abi-Kheythamah, Died in 279 AH

۲-The book “Al-Malāhim” written by  Ahmad ibn Ja’far ibn Al-Monādi, Died in 336 AH

And three other books “Al-Arba’oon fi Al-Mahdi”, “Na’t Al-Mahdi” and “Manāghib Al-Mahdi” written by Abu Na’eem Isfahāni”, Died in 430 AH

Further study:

Mahdawiat in “Sihah Sitta”, P.19

[The Kutub al-Sittah are six (originally five) books containing collections of hadith (sayings or acts of the Islamic prophet Muhammad pbuh) compiled by six Sunni Muslim scholars in the ninth century CE. They are sometimes referred to as Al-Sihah al-Sittah, which translates as “The Authentic Six”. They were first formally grouped and defined by Ibn al-Qaisarani in the 11th century, who added Sunan ibn Majah to the list. Since then, they have enjoyed near-universal acceptance as part of the official canon of Sunni Islam.]