The World War I section of our booklist is very solid, and has a ton of books, with many (but not all) tagged with difficulty, and topic.

I would also express a word of caution, generally, for WWI, as there is scholarship going way, way back, but the historiography as really shifted in recent decades, so there are classic works out there such as Tuchman which are well known and popular, but are considered to have aged terribly by historians of the conflict writing today, and even today popular histories still are often quite attached to an outdated view of the way that just isnt' reflected by more academic works of recent vintage.