A few years back, I had just started a job in payroll and was using a calculator to do things I really should have been able to do in my head. Quite frankly, this made me pretty terrible at that job. I had just figured I was not a "math person" with bad "number sense" and was not very long for that job without some help.
Lo and behold, a supervisor recommended this book when I expressed my concerns and willingness to do whatever it took to become better at my job. With a little work over the next month or so I proved myself entirely wrong at not being a "math person". There's no real magic to calculations in your head, but some techniques are better than others, especially for error-checking. A bit of hard work at drills and memorization can make you that much better. I stayed in the job for 3 more years and when I left I was one of the top people in a large office (200+) in terms of accuracy.
Provided you're not lazy and do the drills in the book, you'll increase your computation time after learning the practical tricks that bookkeepers have used for decades that the academy doesn't know and that your teachers never taught you in school (e.g. the division by 3 check, digit checks, adding from right to left, grouping tens, etc).
No academic jargon or nonsense or high-falutin theory. Just a regimen for improving your mental calculation speed designed for business. It's like exercise. It takes continuous work over time, and it's not always fun, but the results are worth it. Everything worth doing takes work, so beware of any "magic math systems" that instantly make you faster. There are no steroids when it comes to math.
If you're new in a payroll, bookkeeping or other field that requires fast mental calculation, an absolute must have.