Jock was originally introduced in the 1987 Awdry tie-in book - The Island of Sodor - prior to being brought to life properly by Christopher in this new book about the Small Railway.  This was the second book to have the Small Engines as the main focus, and introduces Frank The Diesel as well.

We Need Another Engine

The book opens with the news that the book written about the engines by the Thin Clergyman is about to be published.  There's excitement all round, but disappointment from Frank the Diesel, who discovers that he won't be featuring.

He's cross the next morning, and is difficult with his Driver when he tries to start Frank up.  He does though and starts with a jolt, and ends up smashing into the back of the shed!
However, Frank proves his usefulness again when he rescues Rex's train after he suffers a broken steam pipe, and allows Rex time to be mended.  But despite the Small Controller praising Frank's actions, he knows that the Small Railway needs another engine!

Sticking Power

When Bert feels unwell, Mike and Rex are quite unsympathetic as to his plight and tell him he needs "Determination and Sticking Power".  The fitter promises Bert new pipes over the winter, and lets him in on the rumour about the new engine!  But despite being given a good clean out, Bert soon feels poorly again.
One afternoon, when Bert is due to take his train home again, he loses his tender coupling.  All seems hopeless until the Driver has an idea about "sticking", and decides to use glue to mend the train.
As silly as the plan seems, it works and Bert gets the train home with little difficulty.  Rex and Mike come home to the shed in the evening looking exhausted.  Bert laughs at them and accuses them of having no "Sticking Power" and then tells them about his adventure with the glue!

Jock
Bert is suspicious of something happening in the workshops when he spies a boiler and wheels one afternoon when the doors are open.  The three engines are excited and full of questions when the Drivers come the next morning, and the rumour is confirmed that there is to be a new engine.
A few weeks later, the engine is brought out for tests and is as yet un-named.  The undercoat colour that he is still carrying, reminds Douglas of engines he knew back in the Highlands of Scotland that were nicknamed "Jocks", and suggests the name to the Small engine and Controller.  Both duly accept, and from there on, the name became official!
 
Teamwork

Jock proves a star attraction to the railway and becomes rather cocky.  He becomes cockier still when he is used because of his superior strength to pull a large lorry trailer into the yard, much to the dismay of the others when he boasts about it in the evening!
When Mike is forced to double-head with Jock the following day, he decides to play a trick and cut Jock's big-headedness down.  He gradually cuts off steam after "The Green" and leaves Jock to carry the train on his own.
But Jock realises what's happening and tries to entice Mike into pulling the train himself, by tricking him into going fast.  But Mike doesn't even get the chance when his injector fails and Jock has to pull the train and Mike home again!
At the end of the day, Mike is mended and they both have learned a lesson in teamwork and co-operation.  And looking around him, Jock was glad he was one of the team!