I recently read Winifred Philips A Composer's Guide to Game Music and found it to be a huge help in developing music samples for video games. It offers a hollistic view of the entire industry rather than just hitting on the techniques used in creating video game music loops.
The book starts off with a look at the tools every game composer should have in their bag before venturing into the world of game music. We each have a creative skillset and need to understand it before delving deeper into what we want to do with our skills. It also talks about the levels of psychological immersion experienced when gamers play a video game, and how composer's music affects each level to amplify the affect of the action going on on-screen.
The way themes are explained is very logical and gives you "a-HA" moments that will click and resonate with you through your musical adenture as a game composer. Learning about leitmotifs, liner music, loops music, and interactive songs is a great explanation of the types of music we will be required to create by audio directors, as well. These are all explained in Winifred's book.
The final piece that I found truly helpful was treating your composing like a business. You have to promote yourself professionally (including building a website) and it's not always intuitive to realize this. You must present yourself as the expert and showcase your expertise any chance you can to find more work.
All in all, I found Winifred Phillips's book A Composer's Guide to Game Music to be a comprehensive and extremely useful reference that I will continue to use in my career. I suggest you take a read!