It depends. It would be hard to think of every instance. In your example, it could be reworded to say the same thing...
"contains examples and explanations that will help you improve your.."
"contains examples and explanations that will help you to improve..."
The "to" is not always needed with an infinitive, and, as in the case you mentioned, can be omitted. It would be correct to have "to" before both "help" and "improve," but the sentence becomes awkward and unnatural.