"Read ! It really helps develop vocabulary" just what I was going to say.
Also watch/listen good orators, modern and historical. Mlk, jfk, Peter Ustinov, thatcher (yes I hated her too but she was a very effective speaker), Obama, Churchill, Clinton, Roosevelt, Jackie O, the Greeks (Socrates, Cicero etc), Hilary Clinton, nye bevan, Gloria Steinem...
Watch things like Ted talks or even live parliamentary debates, or mps first parliament speeches.
"Probably a silly question, but any books in particular? Does it matter whether it’s fiction or non fiction?"
Not especially, though I'd err more to the literary than the popular for this. I'd personally recommend political/ideological speeches, well written biographies and autobiographies especially those by orators.
On a more "practical" level, slower - and then even slower was a tip I was given years ago, breathe, enunciate more than usual, pause after tricky words or concepts, don't overly focus on saying the exact words rehearsed of a speech, but understand it well so you can get the thrust/gist across. The audience don't know you missed a "the" in your 2nd sentence.
Practice, record yourself, listen to which words you struggle with and either work on pronouncing or thesaurus up an alternative (I have a slight speech impediment so it's best to avoid certain words)