I've been short of time recently for searching (summer here :-)) and then every book I could find that looked interesting was very long. So I just fell back on 3 long oldie popular novel ones that have sat in my to be read list for ages and that probably most know of even if only through the movies or TV series.
I, Claudius - Robert Graves
https://en.wikipedia.org/wiki/I,_Claudius
https://www.goodreads.com/book/show/18765.I_Claudius
Quo Vadis - Henryk Sienkiewicz
https://en.wikipedia.org/wiki/Quo_Vadis_(novel)
https://www.goodreads.com/book/show/538845.Quo_Vadis
Spartacus - Howard Fast
https://en.wikipedia.org/wiki/Spartacus_(Fast_novel)