Edit:
A couple of other suggestions.
Managing Gigabytes: Compressing and Indexing Documents and Images[2]
Mining of Massive Datasets[3]
Algorithm Design Manual[4]
Network Algorithmics[5]
Neural Network Design[6]
I think all of these fall into the category of "Won't be applicable to everyone, but can be good for those who need this kind of stuff."
[1]: https://github.com/clever-algorithms/CleverAlgorithms
[2]: https://people.eng.unimelb.edu.au/ammoffat/mg/
[3]: http://www.mmds.org/
[4]: https://www.amazon.com/Algorithm-Design-Manual-Steven-Skiena...
[5]: https://www.amazon.com/Network-Algorithmics-Interdisciplinar...
Amazing book. Very readable. I highly recommend it. The book has a section call "War story" at the end of each chapter in which Skiena shares his real life experience of when the contents from that particular chapter came in handy for him.
Go through it. You won't regret
https://www.amazon.com/Algorithm-Design-Manual-Steven-Skiena...
https://www.amazon.com/Algorithm-Design-Manual-Steven-Skiena...
Far more readable than the usual text (Cormen), the first half is a guide on how to select and design algorithms for the problems you encounter, and the second half is a whistle-stop tour of hundreds of well-known algorithms. The tour helped me a lot with X->Y esque issues where I was building bad solutions because I didn't know anything better could exist.
Incidentally, there's a lot more to CS theory than algorithms and data structures, but if you're asking on HN for a generic CS theory book, I reckon it's most likely an algorithms and data structures book that you're after.
As others have mentioned any books on Data Structures & Algorithms are a must. [4], [5], [6]
However in my opinion, trying to understand CS fundamentals without undergoing some sort of formal education is a chore. You won't know what you are missing. Going through an established approved syllabus will give you a fuller understanding. But that is only if you are interested in the long haul.
There are a number of MOOCs that may fit the bill allowing you to slowly gather the knowledge without sacrificing too much time and focus on a day job to keep you going. I feel they are also great value for money for what you get. Some of them are from very reputable names if that is important. [7][8].
Since you have a B.Sc you can do the Masters level but there are also Bachelors level courses. [9]
1. https://www.amazon.com/Graphics-Gems-Andrew-S-Glassner/dp/01...
2. https://www.amazon.com/Graphics-Gems-II-IBM-No/dp/0120644819...
3. https://www.amazon.com/Graphics-Gems-No-3-David-Kirk/dp/0124...
4. https://www.amazon.com/Introduction-Algorithms-3rd-MIT-Press...
5. https://www.amazon.com/Algorithms-4th-Robert-Sedgewick/dp/03...
6. https://www.amazon.com/Algorithm-Design-Manual-Steven-Skiena...
7. https://www.coursera.org/degrees/master-of-computer-science-...
8. https://www.coursera.org/degrees/mcit-penn
9. https://www.coursera.org/degrees/bachelor-of-science-compute...