I don't think they have a Unity version for Linux. I looked about a year ago and they didn't. You can build Unity programs in Windows for Linux, but I don't think there's an actual Unity version for Linux, unfortunately.
Monogame is pretty awesome. I learned game programming in XNA. It's easy enough anyone can begin with it, yet it's advanced enough that there is almost no advanced topic you can't do with it. It grows with you. It gave me the foundation to learn DX11 and then OpenGL.
The XNA 3 books were especially awesome. Although, you're going to have problems because MonoGame is like XNA 5.0 and there was an enormous change in the way graphics were handled between XNA 3 and 4. So, XNA 3 code will probably not work exactly as written in MonoGame. You can convert the code, but it would be a very difficult task for a beginner. I know I could because I've worked in all 3. But until you know what you're looking at it would be rough. But those XNA 3 books have a ton of game programming wisdom in them you may not be able to find anywhere else.
Check out RB Whitaker's stuff. He has some great MonoGame tutorials. He's been doing this since at least the XNA 3 days. I have some XNA stuff on my website although I've really been moving away from that over the past few years and focusing on OpenGL. Most of that has XNA 4.0 projects linked where you can see the entire source code. With MonoGame, you could maybe download the code and copy and paste it into a MonoGame project. There's likely few changes in the code itself. I actually converted two of the projects to MonoGame myself, the Matrices in Motion example and the Textured Grid example. Plus, the project files contain any art assets necessary.
