Well, this is a question I've seen pop up a lot, and honestly, the continuity between the novels and the manga for 'Break Blade' is messy. They're different beasts. The novels by Yunosuke Yoshino are the original source, but they were never fully translated. The manga adaptation by Sorayama Yōsuke is what most people know, and it deviates significantly after a certain point.
If you want the full, intended story, you'd ideally start with the novels, but good luck finding them in English past the first volume. So, practically, everyone starts with the manga from volume 1. The six-part anime OVA series follows the manga's first arc closely, so you can watch that alongside the early volumes. After around volume 7 or 8 of the manga, the plot takes its own road, diverging from where the novels left off. My advice? Just stick with the manga from start to finish; it's a complete, self-contained story with incredible mecha designs and political intrigue, and you won't miss crucial info by skipping the untranslated novels. Trying to splice them together is more headache than it's worth, trust me.