A lot of older books (copyright is 50 years in many countries) are now in the public domain.
So you can find free copies online and often in audio version as well.
Plus if you have an audio version, you can also listen and readalong with the hardcopy.
A lot of older favourites, classics, would be free eg Anne of Green Gables, Tom Sawyer, David Copperfield etc.
Some are still popular today. For children I would recommend The Secret Garden (recent movie version was not that great IMHO...didnt stick with the book)
A Christmas Carol
Robin Hood
Little Women
etc. I once read the entire Little House on the Prairie series. I learned quite a bit about pioneer days in the US. I would have never really learned it otherwise, but as a historical series, its great. People say oh but its racist etc..well it is...but the times it was written just about everyone was.
The tv series which I saw a bit of tended to idyllise it..the books were much more real about the hardships people faced. When I read the one about Almanzo and how he was working on the farm and thats why he didnt go to school half the year when he was 8 years old its kinda like wow many children these days dont even get proper jobs till they are way into their 20s and have spent YEARS in school getting an education. lol
Times have changed!