Reddit mentions of World War I: The Definitive Visual History

Battlefield 1 is fantastic. Here's my personal favorite two - books about guns. No, I'm not a gun nut, I just really like military history. The first one goes more into history, while the second has specifications and technical information. Both are really interesting and make a great conversation starter. The best part is as you can see, they're not expensive at all.

As for history, any of - the Smithsonian's - visual history books are fantastic.

A personal favorite of mine, though, is Timeline. It's a gorgeous art book with a timeline of human history. It really helps kind of put things in perspective, and while it's not photographs, the art is really well done and appealing.

As for mycology, check the website of major arboretums near you. I'm lucky enough to live near the Virginia State Arboretum, and they usually have contact emails and are more than glad to recommend books, activities, and even offer volunteer positions to help you turn your interest into a hobby. I'm not sure myself about fungi books, but your local arboretum (usually on a major university campus) or the Virginia Arboretum should be able to point you in the right direction.

For anything else, you can usually find good art books by googling (Subject) Visual History or Visual Compendium, Visual Compilation, Etc. Visual is usually the key word. Even (Subject) Visual Book usually nets some great results.

Not sure if it's "reading" material per se, but he might be interested in a World War 1 atlas. I can't attest to the offers on Amazon, but this visual history for example seems to cover with visual examples key battles of the war, key figures in the war, weapons and tactics used in the war, personal accounts, illustrated timelines... it looks a lot more fun than just a straight-forward book.

And this atlas (albeit without photo examples and a steep price tag) promises "a comprehensive visual guide to this complex conflict. It plots the exact course of the land, sea and air campaigns in fine detail, enabling the reader to trace the ebb and flow of the fortunes of all sides. With the aid of more than 160 full-color maps, every theatre of war is covered – from the Western Front to Sumatra, from Gallipoli to Galicia to Ghana, from Dogger Bank to Dalmatia, from Romania to Rhodesia, from the Falklands to Togo to Sinai."

Anyway, I think these could be as good and informative a Battlefield 1 companion as a straightforward text.

Ive got this one which might be appropriate https://www.amazon.com/World-War-Definitive-Visual-History/dp/1465419381