Each day, Packt Publishing is releasing a free eBook from their back catalogue. These are usually slightly older books, but mostly usable and valid still. They give a good opportunity to dive into a topic and they will suggest possible alternatives, which are sometimes more up to date.
Today (May 11th) my own book “Unity for Architectural Visualization” is free book of the day, but act fast, as it’ll not last long.
The book was written for Unity 4.x and uses mostly the features from the free version. Since Unity 5.x, most former Pro-features are also accessible in the free version. The biggest value (from my point of view) are the chapters on workflow with CAD or BIM software. You can get better lighting and materials more easily in Unity 5, but the rest of the book (including the scripting section) is mostly valid. I would advice though, to forget about the older Packages from Unity 4. Some of them are replaced with much better and updated versions in Unity 5.
P.S. If you use the above link on another day, it'll lead you to another book. Might be cool, might be less relevant for you.