George Varghese's Network Algorithmics is a good book for backend-oriented people to learn various fundamental CS algorithms.

It presents various CS topics from the perspective of backend optimization, so it's a good book for approaching theoretical CS from a background you already probably understand.

But, ultimately, the best asset for somebody with a CS background is not so much having immediate knowledge, but knowing where to acquire knowledge as necessary. If you have a general idea that for a specific scenario, you can acquire X knowledge in Y resource as you go along, then you're already doing quite well.