Some translated versions of the Upanishads are available widely and read by non dvijas as well. Can I read a Hindi translation?
It is impossible today to know the varna of a person and so Upanishads can be read by all.
(1) Yudhisthira said, " In human society, O mighty and highly intelligent serpent, it is difficult to ascertain one's caste, because of promiscuous intercourse among the four orders. This is my opinion. Men belonging to all orders (promiscuously) begat offspring upon women of all the orders. And of men, speech, sexual intercourse, birth and death are common. And to this the Rishis have borne testimony by using as the beginning of a sacrifice such expressions as -- of what caste server may be, we celebrate the sacrifice. Therefore, those that are wise have asserted that CHARACTER IS THE CHIEF ESSENTIAL REQUISITE. .... WHATSOEVER NOW CONFORMS TO THE RULES OF PURE AND VIRTOUS CONDUCT, HIM HAVE I, ERE NOW, DESIGNATED AS A BRAHMANA.'
(Mahabharata Aranya Parva Section CLXXIX)
(2)The sage Bhrigu said, ‘There is really no distinction between the different orders. The whole world at first consisted of Brahmanas. Created (equal) by Brahman, men have, in consequence of their acts, become distributed into different orders. They that became fond of indulging in desire and enjoying pleasures, possessed of the attributes of severity and wrath, endued with courage, and unmindful of the duties of piety and worship, - these Brahmanas possessing the attributes of passion, - became Kshatriyas. Those Brahmanas again who, without attending to the duties laid down for them, became possessed of both the attributes of Goodness and Passion, and took to the professions of cattle-rearing and agriculture, became Vaisyas. Those Brahmanas again that became fond of untruth and injuring other creatures, possessed of cupidity, - engaged in all kinds of acts for a living, and fallen away from the purity of behaviour, and thus wedded to the attributes of Darkness, became Sudras. Separated by these occupations, Brahmanas, falling away from their order, became members of the other three orders. All the four orders, therefore, have always the right to the performance of all pious duties and of sacrifices. Even thus were the four orders at first created equal by Brahman who ordained for all of them (the observances disclosed in) the words of Brahma (in the Vedas). Through cupidity alone, many fell away, and became possessed by ignorance. The Brahmanas are always devoted to the scripture on Brahma; and mindful of vows and restraints, are capable of grasping the conception of Brahma. Their penances therefore never go for nothing. They amongst them are not Brahmanas that are incapable of understanding that every created thing is Supreme Brahma.
(Mahabharata Santi Parva Section CLXXXVIII )
(3) The famous Rishi Yajnavalkya continued,’ .. All orders of men are Brahmanas. All are sprung from Brahma. All men utter Brahma. Aided by an understanding that. I inculcated this science teaching of Prakriti and Purusha. Indeed this whole universe is Brahma. From the mouth of Brahma sprung the Brahmanas; from his arms, sprung the Kshatriyas; from his navel, the Vaisysa; and from his feet, the Sudras. All the orders, (having sprung in this way) should not be regarded as pilfering from one another. Impelled by ignorance, all men meet with death and attain, O King, to birth that is the cause of acts. Divested of knowledge, all orders of men, dragged in terrible ignorance, fall into varied orders of being due to the principles that flow from Prakriti. For this reason, all should, by every means, seek to acquire knowledge. I have told thee that every person is entitled to strive for its acquisition. One that is possessed of knowledge is a Brahmana. Others, (viz., Kshatriyas, and Vaishyas and Sudras) are possessed of knowledge. Hence this science of emancipation is always open to them all.
(Mahabharata Santi Parva Section CCCXIX)
Yudhisthira says that it is difficult to ascertain a man's caste given widespread intercaste marriages. The sage Bhrigu says that people who are not capable of understanding that all created beings are Supreme Brahma can not be called Brahmanas. The famous Rishi Yajnavalkya says ,’ .. All orders of men are Brahmanas. All are sprung from Brahma. All men utter Brahma. ' If you read through these passages you will see that today there is no way to know the actual caste of any person. Moreover even if you know about it you have to respect every person as Supreme Brahma and that all orders of men are Brahmanas.
Some medieval texts have suggested that varna should be decided by birth in an endogamous group. Hindu scripture categorically denies the idea that Varna is based on birth in an endogamous group.
Maheshwara said, ‘..Neither birth, nor the purificatory rites, nor learning, nor offspring, can be regarded as grounds for conferring upon one the regenerate status. Verily, the conduct is the only ground. All Brahmanas in this world are Brahmanas in consequence of conduct. A Sudra, if he is established on good conduct, is regarded as possessed of the status of a Brahmana. The status of a Brahma, O auspicious lady [Uma], is equal wherever it exists. Even this is my opinion. He, indeed, is a Brahmana in which the status of Brahma exists – that condition which is bereft of attributes and which has no stain attached to it. of human beings in four orders dependent on birth is only for purposes of classification. The boon giving Brahma, when he created all creatures, himself said that the distribution of human beings in four orders dependent on birth is only for purposes of classification.'
Mahabharata Anusasana Parva Section CXLIII