For the Sikh tradition: the Guru Granth Sahib Ji itself, with Sahib Singh's Darpan or a good translation alongside. Bhai Kahn Singh Nabha's Mahan Kosh for reference. Khushwant Singh's two-volume History of the Sikhs as a narrative. J.S. Grewal's The Sikhs of the Punjab for the modern scholarly account.
For the eighteenth century: Rattan Singh Bhangu's Prachin Panth Prakash, Hari Ram Gupta's volumes on the misls, Ganda Singh's document collections.
For the colonial period: the district gazetteers and settlement reports for whichever district you care about, and Imran Ali's The Punjab Under Imperialism for the canal colonies.
For 1947: Urvashi Butalia's The Other Side of Silence, Ritu Menon and Kamla Bhasin's Borders and Boundaries, and then Manto, Amrita Pritam, Bhisham Sahni and Bedi, who did the work the historians could not.
For the literature: Waris Shah's Heer, Bulleh Shah's kafis, Baba Farid Ji's shloks, Shiv Kumar Batalvi's Loona, Gurdial Singh's Marhi da Deeva, Pash.
For everything else: the settlement report for your own district, which is free, and an afternoon with the oldest person in your village, which is also free.
For the general history: the standard English accounts of the Sikhs and of the Punjab, read against each other, because each has an argument.
For the primary material: the district gazetteers and the settlement reports, which are free online and are the most detailed description of the Punjabi countryside that exists.
For the literature: the qissas of Waris Shah, Hashim and Peelu, the Sufi kafis, and the twentieth century novelists. Most of it is only in Punjabi, which is the whole point of learning to read it.