I am very sorry for my late response! I was offline due to sailing camp.
I am glad you like this symphony. Thank you for the great compliment about prefessional orchestration!
As an adolescent I don't have much money for theory books like Samuel Adler's The Study of Orchestration. I have never read it, but most of the professional composers recommend it. So if you can afford to buy it, buy it!
A half year ago I started to study orchestration (of which I had never heard before... oops) and I bought the book The Anatomy of the Orchestra by Norman del Mar. I really like the book, although it isn't a manual for orchestration, rather a treatise:
''A detailed treatise and study of orhestral practice, written for conductors, players, students and for everyone concerned with the performance of orchestral music. The main point of the book is to consider the mainfold difficulties which arise in translating printed scores into living music; it is not a manual of orchestration.''
The other two books about orchestration are Cecil Forsyth's Orchestration and Essential Dictionary of Orchestration by Black and Gerou.
The first one tells the history, usage, characteristics and a lot more about all classical instruments used before World War II. A second edition was published in 1935. Modern instruments like synthesizers are not discussed, because these didn't exist yet.
The latter is a small book that is easy to take away in your bag. All instruments are described in points and sorted in alphabetic order like in a dictionary. When you have read Forsyth's and / or Adler's book this little booklet is very useful, because you can easily find the information you are looking for. Only this book will not be enough. It's just a support.
As a Dutch pupil, reading English books is very hard for me and it costs a lot effort and time. There are no Dutch books about orchestration, but maybe there are some Spanish translation for you, assuming that you are a Spanish speaker. Watching videos on YouTube about orchestration is still intensive for me, but not as much as reading high level books. Some great channels:
OrchestrationOnline (!): https://www.youtube.com/channel/UCSt_ZRe_mla4tRgYC_GNElQ
A gold mine for all composers. This is my biggest source of information about orchestration.
JJay Berthume (more film music and orchestration): https://www.youtube.com/channel/UCaBpxvYSPRS6Gw3FJKA3jgQ
Art of Composing (also orchestration): https://www.youtube.com/channel/UCCcGEQFxpDPt7qh5adqgMzQ
Furthermore there is a channel called Smalin (https://www.youtube.com/channel/UC2zb5cQbLabj3U9l3tke1pg). Malinowski, who is the creator of the Music Animation Machine, posts videos with various fundamental ideas. In some videos all instruments are presented in different colours and shapes and this makes it very easy to see how the composers double instruments or let them play unisono. One example of a video with orchestration as fundamental idea:
The last advice is to write, write and write. Exercise is everything, the more you orchestrate, the better you become!
Thanks again for your compliment and I hope that this is useful for you!
¡Saludos!
Maarten