Reading self improvement and motivational books can have a life changing impact IF you actually use the ideas and strategies you learn. So many people read books and attend seminars but never really commit to taking long term action and as a result, their life doesn't really change and they find themselves attending more seminars and reading more books.
If you're reading these types of books just to read them, then that's fine, continue on. If you want to actually produce results however, nothing beats a plan of action followed by actual action. It's much better to read just a handful of books and consistently apply what you learn than to read hundreds and only take action when you feel like it.
Results in life comes from what you do on a daily basis. If you've gotten into the habit of reading self help books then add onto that the habit of following through and taking action. If you pick up a great idea, even if you're not done with the book yet, apply it. Your goal isn't to finish books; your goal is to gather ideas and apply them in order to achieve the results that you want in life. While you're reading, take notes. Underline and highlight things that jump out at you. Those empty columns isn't just to fill up space. Use them to write your thoughts.
Of course, if you aren't reading these types of books and you aren't already getting the results that you want, then it's time to get into the habit of reading them even if it's just for 15 minutes a day. Just remember, action trumps knowledge. Knowledge without action is pointless.