Aviation and train books for an engineer

My girlfriend's birthday is coming up, and she's a massive transport nerd, especially for planes and trains. I was wondering if any of you have any good suggestions for books on that topic? Specifically, she is interested in commercial aviation (not military), and would be interested in a book on European passenger trains, particularly Germany and the UK. She loves Berlin's public transport system and the history of train travel in Britain.

The main problem I've been having is that she has an engineering degree and I don't, so it's hard to find books which have the appropriate level of detail! She would be bored by a pop science book, but I don't want to get her a textbook, so something with an intermediate level of technical detail would be great. Anything evocative of the YouTuber Mentour Pilot would be a big hit.

Thanks for the help!