There are many tutorials and projects to start with an Arduino on the internet (and on this blog ?) but nothing replaces a learning book to start well. Here is a selection of books that will guide you step by step in the discovery of this platform of learning in electronics and fantastic programming.
Some authors have addressed the general aspects of I/O programming and use. Other works are oriented projects with practical cases of applications: connected objects, sensors, actuators, examples of use in home automation DIY. For this type of book you will need to buy components kit.