Question

Which of the following sentences is correctly punctuated?
I have taken notes on your book however I still have questions.
I have taken notes on your book, however I still have questions.
I have taken notes on your book; however, I still have questions.
I have taken notes on your book however; I still have questions.

Answer 1

3rd sentence is correctly punctuated

Answer 2

Here is the correct punctuation of the given sentence: 
I have taken notes on your book; however, I still have questions.
The word 'however' is a conjunctive adverb that is needed to connect two independent clauses into a compound sentence. As a rule, semicolon always precedes the conjunctive adverb so do not confuse it with one more comma before a conjunctive adverb.