The questions asked are:
- What scriptures to read that focuses on spirituality?
- What is the definition of spirituality in Hinduism?
- How to become spiritual according to Hinduism?
As the questions are related to Hinduism, the order of the questions, in my opinion, are to be re-arranged as follows:
- What is the definition of spirituality in Hinduism?
- What scriptures to read that focuses on spirituality?
- How to become spiritual according to Hinduism?
1. Definition of spirituality in Hinduism
The dictionary meaning of SPIRITUALITY is the quality that involves deep feelings and beliefs of a religious nature, rather than the physical parts of life.
https://dictionary.cambridge.org/dictionary/english/spirituality
This definition applies to all human beings, irrespective of the religion/way of living they are following. Hence, there will be no separate definition of SPIRITUALITY for the persons following Hinduism.
Vaishnavism, Shaivism, Saakteism, etc, are religions, being practiced by people under the name of Hinduism aka Sanaatana Dharma.
Buddhism, Jainism, Christianity, Islam, etc, are Religions.
All religions are insisting of prayers to God at certain intervals, reading of respective Holy text, following respective customs.
Religion and SPIRITUALITY are different.
The purpose of any religion is to show direction towards spirituality. We have to understand that spirituality has no affiliation to any religion.
The spiritual practices make a human being realise, not understand, the God within oneself.
2. Scriptures to read that focuses on spirituality
The core values of SPIRITUALITY advocate self enquiry.
Vedas, which are stated to be oldest and divine, advocated self enquiry for realising the TRUTH.
https://books.google.co.in/books?id=2rvQCgAAQBAJ&pg=PT239&lpg=PT239&dq=self+inquiry+in+vedas&source=bl&ots=ruAnKLJIgK&sig=ACfU3U1GwHa1W2MDu92a3HXorHKkshO08Q&hl=en&sa=X&ved=2ahUKEwjFpvuny_fiAhXNfX0KHVROBgE4ChDoATANegQICBAB#v=onepage&q=self%20inquiry%20in%20vedas&f=false
https://en.wikipedia.org/wiki/Neti_neti
If you are well versed in Sanskrit, you can go through Vedas, which contain pure SPIRITUAL aspects, Upanishads, etc.
You can go through the Talks with Sri Ramana Maharshi, which will give you an idea about SELF ENQUIRY.
https://www.amazon.com/Talks-Ramana-Maharshi-Munagala-Venkataramiah/dp/8188018074
And, many other schools published their own literature on SELF enquiry.
3. How to become spiritual?
It is a complicated subject. It depends on so many factors.
For Example;
You are running a manufacturing company, which is running successfully. After some time, you got vexed with business, and want to close down your company.
Is it possible overnight? No.
Either you should get a buyer to sell it off as is where is basis, or you should start slowing down your business activities, and finally wind up the company.
You have to reduce your liabilities and sell off assets slowly, while reducing the raw material usage in manufacturing slowly. It takes time, but surely can be done.
When you embark upon SPIRITUALITY, you cannot overnight renounce the world, as Gautama, the Buddha did. It is not possible in every case.
You have to reduce your desires slowly, discharge your responsibilities fully, engage yourself in SELF ENQUIRY or meditation regularly.
Sri Krishna said in B.G.
ध्यायतो विषयान्पुंसः सङ्गस्तेषूपजायते।
सङ्गात् संजायते कामः कामात्क्रोधोऽभिजायते।।2.62।।
In a person, meditating on sense-objects, attachment or them is born in succession; from attachment springs passion; from passion arises wrath.
क्रोधाद्भवति संमोहः संमोहात्स्मृतिविभ्रमः।
स्मृतिभ्रंशाद् बुद्धिनाशो बुद्धिनाशात्प्रणश्यति।।2.63।।
From wrath delusion comes to be; from delusion is the loss of memory; from the loss of memory is the loss of capacity to decide; due to the loss of capacity to decide, he perishes outright.
So it is the desire, whichever form it might be, that spurs an activity in the individual.
Desire can be towards copulation with opposite sex, towards acquiring riches, acquiring fame, etc.
It does not mean that desire is bad in toto, but excessive indulgence in the desires is bad.
It is but NATURAL to have an inclination towards copulation with opposite sex, but over indulgence will make a person weak physically and distract mentally one from SPIRITUALITY.
Acquiring riches for discharging one's responsibilities is good, but avarice should be avoided.
Final word:
Any SPIRITUAL practice without an assistance from a Guru in physical form or the invisible God, will not allow one to reach one's goal.
A Guru will teach, warn, guide at an appropriate time and give push, at an appropriate moment, to the disciple towards SELF enquiry. Without the assistance of a Guru, one cannot understand whether one is progressing SPIRITUALLY or not.
For example:
People will be performing journey in Aeroplane. The passengers do not know the height at which the Aeroplane is flying, how much time will it take to reach the distance, etc.
Only a pilot can tell this, but he is placed in a separate cabin, and will communicate with the passengers at an appropriate time.
SPIRITUAL journey is like flying in a Aeroplane. Guru is like a pilot.
One has to wait for the guidance of the Guru.