Col Sydney Long Jacob was my great grandfather. I have read this book and you can see what a deeply spiritual man he was, who had a love for God and people.
I would love to find out more about his role within the Plymouth brethren church.

What a privilege to have a man like the colonel in your family; surely a reason to "remember those who spoke the Word of God," and an encouragement "to follow their faith" (Heb 13:7).
You may know of the page in Stem Publishing - https://www.stempublishing.com/authors/Jacob.

See also Peter Conlan's valuable Chapter 13 ' "What are you doing for Christ in Birmingham?" : The History, Strategy, and Legacy of a Missional Brethren Assembly Founded by Alexis Jacob in 1924' in N. Dickson, T.J. Marinello, [eds.], 'The Brethren and the Church' (Glasgow [BAHN] 2020) pp.218-27.