John Buchan 
A Prince of the Captivity [EPUB ebook] 

The hero of the novel is Adam Melfort, who marries young to a beautiful but mindless socialite who cannot return his love for her. When she forges her wealthy uncle’s signature on a cheque, he takes the blame to save her family’s name, and is jailed, losing his army commission in the process. He allows her to divorce him so that she can remarry someone of more similar mind. Released from gaol during World War One, he is recruited as an undercover agent behind enemy lines in Belgium, and later leads an expedition to Greenland to rescue a wealthy American millionaire explorer whose own expedition has met disaster.