When you first saw that title, did you think -
a. If I wanted to read books I would of done so?
b. Whoever wrote that title should have been strangled at birth?
c. 'Learnt' isn't a real English word, it should be 'Learned'?
If you answered-
a. Stick around, we need to talk.
b. You won't learn anything from this short article.
c. Sorry, but you're wrong! 'Learnt' is a perfectly valid English word, although if you live in the USA you might have been taught otherwise. Look it up, then go back and choose either a. or b.
OK, now that everyone who chose b. has lost interest and gone away, we are alone and can speak privately.
I'm sorry to have to tell you this, but the expression 'should of' does not exist in the English language, although more and more people are now using it. It is a corruption of 'should've', which actually means 'should have'. The same applies to 'could of', 'would of', and so on.
If you use 'should of' instead of 'should have' it's a pretty safe bet that you spend much more time talking to people, texting, and using FaceBook than you spend reading real books (magazines don't count in this context). When you read books you see the word written as "should've", and that ought to make it obvious that it means "should have", but if you rarely read you learn by hearing only, and so "should've" is learnt as "should of", because, to be fair, it does sound quite like that.
Usually there's absolutely nothing wrong with learning by hearing, in fact we all spend much of our time doing it, but in this particular case it can be a problem. Without even being aware of it, every time you use 'should of' you are broadcasting your lack of knowledge of the English language. It's probably no big deal if you are simply talking to friends, but how about if you are writing to a prestigious company to apply for a job, and you really, really want that job?
I assume in a few years time the use of 'should of' will become accepted as normal, in fact it's probably inevitable. Not long ago one of my greatgrandchildren brought home a school report, and at the bottom of one page the teacher had written "Could of done better if he had worked harder". What hope is there when even the teachers themselves don't have a clue?
I give up...