While modern textbooks often weigh 10 pounds and cost a semester’s worth of coffee, there is a gem from the mid-20th century that deserves a spot on your hard drive:
Because the book is in the public domain in many jurisdictions (depending on the printing year) or widely available through academic archives, you can find clean scans online. Ecuaciones-diferenciales-elementales-kells-pdf
You can download a high-quality scan of "Elementary Differential Equations" by Kells here: [Link to PDF] The Verdict Does Kells have fancy 3D graphics? No. Does it have a thousand online homework codes? Absolutely not. But does it teach you how to solve a differential equation better than 90% of modern textbooks? Yes. While modern textbooks often weigh 10 pounds and