You are asking some very important questions. And your skepticism is a sign of a scholar in my book. ;o)
I agree with you that the definition provided by your textbook falls short.
Emotion is a huge topic. It might help to narrow the scope of your question.
I also suggest carefully defining a word like "feelings". For example, how are feelings and emotion different? (if they are).
Here is an article that will give you an introduction to contemporary research on emotion:
Critchley, Hugo D. and Sarah N. Garfinkel. "Interoception and emotion." Current Opinion in Psychology 17 (2017): 7–14. [Open access]
Abstract: Influential theories suggest emotional feeling states arise from physiological changes from within the body.
Interoception describes the afferent signalling, central processing,
and neural and mental representation of internal bodily signals.
Recent progress is made in conceptualizing interoception and its
neural underpinnings. These developments are supported by empirical
data concerning interoceptive mechanisms and their contribution to
emotion.
Fresh insights include description of short-term interoceptive effects
on neural and mental processes (including fear-specific cardiac
effects), the recognition of dissociable psychological dimensions of
interoception, and models of interoceptive predictive coding that
explain emotions and selfhood (reinforced by structural anatomical
models and brain and experimental findings).
This growing grasp of interoception is enriching our understanding of
emotion and its disorders.
Wikipedia has a good article on interoception.
