There are no English biographies on Emperor Yongzheng as far as I know.
To the uninitiated, he is an unimpressive emperor compared to his father, Kangxi, and his son, Qianlong.
But the truth is that during his short reign of 12 years and 7 months, he was a hard-working Emperor.
He refined the memorial system of communication that allowed him to have secret communications with far-flung court officials.
A good book that reveals that he was a hard-working and brilliant emperor is Treason by the Book by Jonathan Spence.
The book Passage to Power Kang Hsi and his Heir Apparent by Silas HL Wu, 1979 by Harvard, narrates the dilemma faced by Kang Hsi throughout his long 61-year reign to select a worthy heir out of 36 sons from 40 consorts.
His first choice, which was his oldest son, Yin Jeng, was anointed as heir, discarded, and reanointed again only to be ultimately discarded in favour of the 4th son, Yin Zhen (era name Yongzheng).
Coincidentally, Yongzheng was also succeeded by his 4th son, Qianlong.
Passage to Power also narrates the subsequent struggles between Emperor Yongzheng and his siblings for control of the throne.
This book makes fascinating reading. Highly recommended.