Joel Osteen's book, "I Declare" is sorely based on an unbiblical teaching that is seeping into the church today - the Word of Faith teaching.
Word of Faith (often called "blab it and grab it") tends to have 3 main tenants: Healing, Prosperity, and Positive Confession.(The "Word of Faith" teaching is closely related to the "Prosperity Gospel", though they aren't the same thing.)
This book, "I Declare" is based most squarely to Positive Confession. Positive Confession states that because God created the universe simply by speaking it into existence, therefore that must mean humans are also endowed with the ability (power) to speak things into existence. You need money? Speak it into existence. You need better health? Speak it into existence.
This is plainly unbiblical and an obvious non-sequitar. Now, God clearly creates the universe in Genesis 1, and His creation is preceded by His speaking ("let there be light"). But just because God has the power to create things by any means He chooses (which would include by the mere utterance of a word) that doesn't mean that humans have that same power. God is omnipotent. Does that mean humans are? God is omniscient. Do you know everything there is to know? God is omnipresent. Are you present everywhere?
Thus, you see, by making a "Positive Confession" (by reciting a promise of scripture, for example) this generates the power which enables those things to come into fruition. Thus, your words have "power", analogous to the same creative power as God in Genesis 1. You are a "little god". You want something? Speak it into being.
Friends, only God has the power to speak things into existence.
Don't waste your time with this unbiblical book.
Far better off reading Mere Chrisitanity or The Screwtape Letters by C.S. Lewis, or Desiring God or Don't Waste Your Life by John Piper, or the Reason for God by Timothy Keller. In particualr, a Biblical alternative to this book about the power of God's word for us (the sword of the spirit - Eph 6) would be John Piper's Future Grace.
This book is really great especially for new believers as it reminds us of Gos's faithful promises. I read it daily and have purchased a copy for my daughter who is enjoying it as well. It's very inspirational and is in small digestible portions.
Someone in leadership from our church shared one of Joel's declarations from this book and it give me much encouragement and excitement from God's Word that I purchased one and then 5 additional in which to bless others.