I gravitate toward books that treat the mecha as a character rather than just a weapon, so my recommendations lean into that. 'The Murderbot Diaries' by Martha Wells is an obvious favorite, though it's more about a cyborg security unit than a piloted frame. The psychological tension of a construct fighting its own programming is the core of the mecha experience for me.
For traditional, cockpit-and-controls stuff, 'Mecha Samurai Empire' by Peter Tieryas is a blast. It's an alternate history where WWII was fought with giant robots, and the sequel, 'Cyber Shogun Revolution,' gets wonderfully weird. They're not the deepest books, but the action sequences are tactile and the world-building is delightfully over-the-top.
If you want something with more literary weight, try 'All You Need Is Kill' by Hiroshi Sakurazaka. It's the novel that inspired 'Edge of Tomorrow.' The powered armor is visceral, a tool of grinding attrition, and the loop narrative explores the mental toll of piloting in a way most stories gloss over. The mecha feel heavy, clunky, and desperate, which I find more compelling than sleek, invincible suits.