All of the books written by Ustadh Mahmoud Mohammed Taha were in Arabic. The mother book Ar-Rsisalah Athaniya Min Al-Islam {The Second Message of Islam} has been translated into english by Dr. Abdullahi An-Naiem. You can obtain a copy from Barnes & Noble or from Amazon.com.
There are a few english booklets written by the Republicans, the followers of Ustazh Mahmoud Mohammad Taha, however Ustazh Taha himself rarely ever used the language despite his appearant command as you would see in a letter he wrote in 1963 answering a few questions presented by an American researcher from Harvard University.
To access the full library in Arabic click here, otherwise you can browse through the following few English books: