I wanted to recommend Hanes Yn Y Tir gan Elin Jones. It’s a history of Wales aimed probably at 13-14 year olds.
Unfortunately it’s not for beginners as the sheer breadth of new vocab is huge. It covers the history of Wales from standing stones, the smelting of metal and hillforts up to the implementation of a sewage system, workhouses and almshouses, the founding of the NHS and everything in between. It’s also written in what I’d call “textbook” language with - wyd passives etc.
But for all that, if you like history or if you prefer non-fiction then I’d get hold of a copy and dip in and out of it as you fancy. If you know a fair amount of history then general knowledge will fill some of the linguistic gaps and there are plenty of images to give you a clue.
Each topic is only a couple of paragraphs long so you don’t have to read the whole thing end to end like you would a novel.
Finally, independent of language I thought it was a great review of the people, places and events
that have shaped Wales’ history.