Take photos of 5–10 important pages from a library book. Use or Adobe Scan to convert to searchable PDF. Keep only for revision – don’t share.
While there are many texts on the market, the Singh and Sapre approach is often preferred for several key reasons: singh and sapre communication system pdf free 89 better
Also try searching for on Google Scholar or arXiv.org – many professors post excellent free notes. Take photos of 5–10 important pages from a library book