In the UK the book is titled "The Godfather:The lost Years"

I've just noticed that although these books have the same release date, "The Godfather Returns" has 448 pages and "The Godfather: The Lost Years" has 400 pages! Different books? or different versions of the same book?

As seen here - Amazon.co.uk

It has this synopsis...

The Godfather: The Lost Years takes place in the years 1955-65, but it is built upon the story of that 'year of delicate political manoeuvring' - and how, in winning the battle of that year, Michael Corleone set the stage to lose the war: the war to make the Family legitimate, the war to keep the Corleones supremely in power, the war to stay true to his father's wishes, the war to give not just his Family but his family a safe and happy life. The Godfather: The Lost Years is not just a sequel. A magnificent novel in its own right, by an acclaimed young American novelist, it traces: the nexus of ambitious, audacious decisions that Michael Corleone implements, their ultimate failure, and, after the Family's literal and figurative years in the wilderness (of Las Vegas), Michael's literal, physical return to New York, and his attempts to regain control there.