"couldn't" is only used to refer to actions that were not performed because of impossibility or inability in the past.
I couldn't come to college last week (even if I wanted to).
means "I was not able to come to college last week" or "It was not possible for me to come to college last week."
Instead, "couldn't have" can express other meanings. I will use some other examples for you to understand the meaning and then your own example explained:
- Inference or deduction of past impossibility:
• George Washington could not have known Abraham Lincoln – they lived at different times.
• As a young woman who had never given birth before, she couldn't have known what the experience of childbirth would be like.
You say you saw me in college last week but I couldn't have come to college because I was abroad last week.
• The team couldn't have won the game if they had not trained so intensively.
I couldn't have come to college if I hadn't given me a ride. (But you gave me a ride, and I was able to attend.)
In addition to its use in past hypothetical conditional sentences complete with a main and a conditional clause, "couldn’t have" is also found in incomplete conditionals in which the if- clause is merely implied, for example:
• I couldn't have done it without my family [if I hadn't had my family].
• The timing couldn't have been worse [even if circumstances had been different].
• I couldn't have written a better essay [even if I'd tried to do it].
I couldn't have come to college [even if I had wanted to].
With actions that involve some ability or skill, "couldn’t have" expresses past hypothetical inability, similar to wouldn't have been able to do sth or wouldn't have been capable of doing sth:
• I wish I had written Sunset Park, but I couldn't have, because no one can write like Auster.
As regards your last two examples:
"This book could not be written without your help" means that it was impossible to write the book because the other person did not offer or provide his/her help and, as a result, the book WAS NOT written.
"This book could not have been written without your help" means that, thanks to the other person's help, it was possible to write the book, so the book WAS written.