There are many aspects to how you write your book, but an important one is “point of view”, or POV.
Before you begin the actual writing, it is a good idea to decide who is telling the story. The most common, and most popular in fiction, is written in the third person, i.e. “Lyari picked up his sword and moved slowly towards the dragon. He wondered if his months of training had really prepared him for this encounter.” The writer, through Lyari, is the one telling the story, and the writer is omnipresent or omniscient; in other words, the writer can see from not only Lyari’s POV, but also everyone else’s point of view what is happening. The writer will use the pronouns “he”, “she”, “him”, “her”, “it” throughout the book. This form of writing will give the most leeway in switching from character to character since the writer can see everything from his or her own perspective, but the characters will not necessarily see everything as the writer does.
If Lyari is speaking in one chapter, I can only write about what he is seeing and experiencing in that chapter. If I need to see and experience something from the dragon’s POV, then I need to switch in the next chapter, making it clear that I am now looking through dragon eyes at the approaching Lyari. Lyari will not be privy to the dragon’s thoughts or feelings, unless he is also telepathic, which, in fantasy, is a possibility. But you need to make it clear from the outset that Lyari is telepathic or has ESP. Maybe the dragon doesn’t know this and is surprised when Lyari counters his moves!
Writing in first person, “I”, “we”, “my”, “our”, etc. is used mainly in autobiographies or books in diary form, but can be used in an exclusive POV for almost any kind of book. Many are put off by this so be wary before using first person in a general way. It can be tricky! If your dragon needs to express his viewpoint then you will need your “I” character, and in the above case it will be Lyari, to be present, otherwise it will not work.
It is better to be consistent in your pick of POV. Do not make the mistake of using first person POV in one sentence, then suddenly switching to the dragon’s POV in the same breath. If you need to switch, then I would use the first person POV for both Lyari and the dragon, but in separate chapters or segments, if possible. This can be confusing to a reader, so I prefer to use third person POV throughout, or first person POV throughout.
Remember that the person or character whose eyes you are looking through to tell the story is also the character to whom the reader will relate most to. Consistency is the key. Even if you are using an omnipresent POV in third person, which I do in my books, I telegraph switches from character to character in different segments. For example, if I was writing a book about Lyari the elf, I would make it clear that it is Lyari that is speaking, and whose thoughts I can hear, whose eyes I (and the reader) are looking through. Then, if I want to switch to the dragon’s POV, I will be clear that it is now the dragon whose eyes I am looking through, and whose thoughts and feelings I can determine. This method gives the writer the most leeway so there are no unpleasant surprises or confusion for the reader.
It is fine to switch back and forth with POV, but be careful in doing this. Be consistent, watch for sudden changes of POV. Are they going to confuse the reader? If so, you may want to reconsider or make it abundantly clear who is telling the story. I think of myself as the secretary, taking down dictation from many different characters who all want to tell their story and be heard. I remember to keep them separated as much as possible, and ensure the reader knows exactly whose point of view I am now writing about.