There's also a booklet called 'The Mersey Tunnel - The Story of an Undertaking,' which was published for the Mersey Tunnel Joint Committee and tells the story of the construction. I think it was originally brought out soon after the tunnel opened.
I have two copies of this: 64 pages, foolscap quarto (slightly larger than A5). No publication information, but in one copy there is stuck a compliments slip from the Birkenhead Town Clerk. At the back of the same copy, but not the other, there's a fold-out plan of the tunnel. This copy has a blue cover, the other, a grey cover.