In the past I used Keynote for its simplicity, now I prefer Sketch. It's nice to be able to preview your designs on your mobile devices. These apps are for Mac only though.

* Books

One book I would recommend on design is "The Design of Everyday Things". You'll learn a few useful things about design in general.

Another book I'm reading right now is "The Process of Creating Life". I cannot recommend it yet as I haven't finished it, so I'm just mentioning it. This book might seem like an odd choice at first since it's from the architecture shelf, but it has some interesting ideas nonetheless.

One thing to mention is that the topic for both of these books is not digital design, yet I would argue that the knowledge learned from them is transferable to the digital realm.