“Lewis Carroll” by Morton N. Cohen
This is probably the most well-known biography. It covers Dodgson’s history reasonably well, giving a lot of attention to his mathematical works, and detailing his travels in Russia.
However, there does appear to be an element of “speculation” in areas where only vague diary entries, etc. seem to exist, with little solid evidence. Although that could just be me.
When reading it, I felt that a biography’s reader needs to be careful in completely accepting everything in a work. An author could be focusing their research on what they expect to find; based on their own views, and what was generally believed before they started their research.
This I feel is especially true when dealing with an enigmatic person like Charles Dodgson. Although you were probably already aware of this.
