(selling deeper and wider in an existing client organization)
Focus on improving your sales skills and delivering constant value to your accounts. With that in mind, I would recommend:
"If You're Not First You're Last" by Grant Cardone
"Closers Survival Guide" by Grant Cardone
"Fanatical Prospecting" by Jeb Blount
It depends on where you're starting. I'd start with New Sales Simplified by Mike Weinberg. It's a great primer that teaches what's "possible" in places like this.
Then, I'd really try to get with client selection and balance activities between Salmon and Whales.
You want to find people that can grow, are in healthy categories, and that may take more effort to get the initial sale, but will have bigger opportunities once you're there.
Read these books bro.
Pivot: The Only Move That Matters Is Your Next One
The 5 A.M. Miracle: Dominate Your Day Before Breakfast.
First 90 Days: Proven Strategies for Getting Up To Speed Smarter and Faster
Ps-If you are not a good reader, you can call me and i will guide you all through.
Stay blessed man.