I've enjoyed Andy Hill's book: https://www.amazon.co.uk/Scoring-Screen-Secret-Language-Music/dp/1495073734
Not sure exactly what the dramatic theory is, but its pretty comprehensive :)
TL;DR:
I'll start my response with the most important piece of advice. Learn to write music for films in your own unique way and style. The world already has a Hans Zimmer and John Williams, it doesn't need more. Your own unique musical style and voice is what will make you stand out from the others. You can certainly learn a lot by studying the works of the greats and incorporating elements of their music in your own, but always remember to keep developing your own musical voice. To do this I suggest you write your own original music that is separate from your film/video game scoring work. If a director hears your original music and asks you to score their film you'll have the benefit of being able to write in your one style which can be extremely fulfilling. You said you write with guitar, that's great. That could be a unique part of your sound. Just look at Gustavao Santaolalla, he works almost exclusively with guitars and he's made that sound his own. So again, develop your unique musical voice.
If you want to get better at film scoring the best things you can do is write music to picture. Find a scene from a movie, mute the audio and write your own music to it. If you can find a film that has no music, No Country for Old Men is a great example, than even better since your music will have to take sound effects into account as well. By simply doing this you should be able to pick up quite a lot about film scoring.
There are also quite a few film scoring competitions online and these are a great place do get some practice in. Try to find ones that allow you to use your written score along with the video as part of your showreel/portfolio so that you'll get something out of them even if you don't win. Once you feel confident that you know what you're doing you should understand that getting a usable portfolio piece out of these competitions is more important than winning. It's your portfolio that'll get you work, not how many sample libraries you've won/bought.
You can certainly compose music on the guitar, but you will eventually need to input notes into a DAW using a MIDI keyboard so try and get some basic proficiency on the keys. You should start by learning to play the following:
You don't need to be at the level of a professional pianist. The main thing is getting to a point where you can play a melody with your right hand whilst playing chords with your left hand.
In terms of music theory you should try and learn some basics. You can go much further but these should get you started
The most important thing is to know what musical and emotional effect different elements of music have on a listener. Learning theory is really just expanding your composer's tool kit and giving you more musical devices to use in your own writing. So the more theory you know, the more ways you can go about creating an emotional response in a listener.
Finally I'd recommend reading some books. With the internet these can be easily overlooked but they're really a gold mine of information. Note that most of the film scoring books out there focus on writing and arranging for the orchestra and include musical notation of score cues, so you'll need to be able read sheet music and have a decent grasp on music theory to get the most out of it. Also note that these books won't show you how to use a DAW, setup microphones or use a MIDI keyboard so if you're wanting to learn these things you're better off looking elsewhere. Instead, these books focus on how past composers have written music for film, so they're a great resource for learning how the pros do it. These books can be a little expensive since they're often used textbooks for university courses, so try and find a library where you can borrow a copy or get a second hand copy if they're a bit pricey. Any of the following books would be worth checking out:
Scoring the Screen: The Secret Language of Film Music by Andy Hill
On the Track: A Guide to Contemporary Film Scoring by Fred Karlin and Rayburn Wright