A book by Paul Shannon,
British Railway Infrastructure Since 1970: An Historic Overview would make a nice present. The title makes it sound a bit dry, but it's an attractive hardback with plenty of interesting pictures and you can usually find it at a big discount compared to cover price. Gives a sense of the changes to the railway over the last 50 years or so. The author has written numerous good quality railway books, mainly about freight, and is very knowledgeable.
https://www.amazon.co.uk/British-Railway-Infrastructure-Since-1970/dp/1526734796/
Some other publications that might be useful to learn more about the industry:
A bookazine called
The Changing Face of Rail Freight is very good, excellent value and has lots of information on freight with loads of pictures of wagons and trains, showing the different commodities conveyed. Captions are very informative, good to browse through.
https://www.classicmagazines.co.uk/product/5576/bookazine-the-changing-face-of-railfreight
The previous bookazine focuses on the freight itself, this one -
Heavy Freight! Diesel and Electric - would be a good companion as it focuses on the loco fleet used to move it. Attractively presented with lots of photos; suitable as a "beginners guide" to freight locos without going into excessive technical detail.
https://shop.kelsey.co.uk/product/railways-of-britain-16-heavy-freight-diesel-electric
The ABC Rail Guide already mentioned in this thread should give plenty of info on the passenger operators
For some background detail on railway operations, it would be worth picking up these 3 pocket-sized ABC books second-hand (they should be cheaply available on Ebay). Two of them were published back in 1999, but even so they give a useful insight into how the railway works:
ABC Railway Operations published in 1999 gives a good general background to how the railway works
https://www.amazon.co.uk/Railway-Operations-Ian-Allan-abc/dp/0711026890
ABC Railway Freight Operations was also published in 1999 and gives an overview of the freight side of things
https://www.amazon.co.uk/Railway-Freight-Operations-Paul-Shannon/dp/0711026602/
ABC Modern Signalling Handbook is a useful explanation of railway signalling (there's an older version of the book which might be cheaper)
https://www.amazon.co.uk/Modern-Signalling-Handbook-Chris-Hall/dp/0711038392/
A "Baker Atlas" is a very useful thing to have:
https://www.amazon.co.uk/Rail-Atlas-Great-Britain-Ireland/dp/0860936813/
A subsciption to a railway magazine, as suggested by etr221, is a good idea. I learnt a lot about the railway from magazine articles when I was growing up. And the Platform 5 spotters books are a classic choice for any beginner rail enthusiast!
In terms of apps, the live signalling maps on Tracksy and OpenTrainTimes are a good way to see what's happening on the network, I have the Tracksy app on my phone.
RealTime trains is another well-known website for train times, both passenger and freight
https://traksy.uk/live
https://www.opentraintimes.com/maps
https://www.realtimetrains.co.uk/
A couple of online railway bookstores that might be of interest, to browse for railway books and see what appeals:
https://www.booklaw.co.uk/
https://strathwood.co.uk/