In short, by not providing an answer key, you are denying the disciplined student the opportunity to efficiently learn.

I agree with you 100%. But let me add this: in most cases, if you're studying with a book that doesn't have an answer key, you can supplement that text with exercises taken from somewhere else. For example, lots of course websites around the 'net post previous years exams / homework with answers. There are also books like Schaum's 3,000 Solved Problems in Calculus[1], The Humongous Book of Calculus Problems[2], 3,000 Solved Problems in Linear Algebra[3], etc.

Also, with books that are used as textbooks, and that provide an answer key but only to instructors... if you aren't averse to violating copyright and using certain pirate websites, those "instructor only" answer keys can often be found.

[1]: https://www.amazon.com/Schaums-Solved-Problems-Calculus-Outl...

[2]: https://www.amazon.com/Humongous-Book-Calculus-Problems-Book...

[3]: https://www.amazon.com/000-Solved-Problems-Linear-Algebra/dp...

1. Khan Academy

2. Youtube (see: Professor Leonard, 3blue1brown, Gilbert Strang, etc.)

3. Books like:

https://www.amazon.com/Humongous-Book-Calculus-Problems-Book...

https://www.amazon.com/Calculus-Practice-Problems-Dummies-On...

https://www.amazon.com/Schaums-Solved-Problems-Calculus-Outl...

https://www.amazon.com/gp/slredirect/picassoRedirect.html/re...

4. And maybe not as calculus specific, but doing Project Euler problems might also be useful.

A lot of times you can find old problem sets, quizzes and tests on the course websites for past sections of courses. Just google something like

https://www.google.com/search?q="linear+algebra"+problems+si...

https://www.google.com/search?num=50&newwindow=1&q=calculus+...

https://www.google.com/search?num=50&newwindow=1&q=calculus+...

or variations on that theme. There's a ton of stuff out there.

If you want a print book, check the various "Schaums Outlines" books, or books in the "For Dummies" series with "Workbook" in the title (ex, "Calculus Workbook for Dummies", etc.) There's also the "Problem Solvers" books and those "Humongous Book of X" books. For example:

https://www.amazon.com/Schaums-Solved-Problems-Calculus-Outl...

https://www.amazon.com/Humongous-Book-Calculus-Problems/dp/1...

https://www.amazon.com/Calculus-Workbook-Dummies-Mark-Ryan/d...

https://www.amazon.com/Calculus-Problem-Solver-Solvers-Solut...

and so on...

This excerpt -

"what he really liked was solving problems. The text of each chapter was just some advice about solving them. He said that as soon as he got a new textbook he'd immediately work out all the problems—to the slight annoyance of his teacher, since the class was supposed to work through the book gradually."

is literally me. I did that. Every year at my school I did exactly that. Once I actually turned in my solutions and my math teacher was quite upset because she didn't know what I'd do for the rest of the year in her class. She thought I was being arrogant and I should take in the material slowly, not swallow it all like a whale. But I wasn't arrogant or anything, because unfortunately this skill didn't transfer to the rest of my classes. I wasn't particularly good at history or physics or anything else, only math. Even now, I have tons of Schaums at my home. Like this one - http://www.amazon.com/Schaums-000-Solved-Problems-Calculus/d... I work problems in it just because it is a craving - I simply have to solve it. Sadly, society doesn't pay for this sort of addiction. I have been a professional programmer for the past 2 decades to pay the bills, but I secretly hate programming, debugging, programmers, git, the whole enterprise - just seems so stupid & futile. But hey, atleast I can spend my salary on Schaums.