So let me get this straight.
Book A says "This is a most important lesson which we will have to study very carefully".
Book B says "This is the most important lesson which we will have to study very carefully".
1. The word "most" should not be capitalised.
2. I don't like either sentence without a comma after "lesson".
3. They are both grammatically acceptable but they have different meanings.
Additionally, please give your threads more useful titles. Titles should include some or all of the words you are querying. A good title for this would have been "This is a/the most important lesson".
Student or Learner