Ten e-books founders actually cite as useful. Most are free or under $10.

1. Traction — Gabriel Weinberg & Justin Mares
2. The Mom Test — Rob Fitzpatrick
3. The Personal MBA — Josh Kaufman
4. Paul Graham's Essays (paulgraham.com, free)
5. High Output Management — Andy Grove
6. Zero to One — Peter Thiel
7. The Hard Thing About Hard Things — Ben Horowitz
8. Indistractable — Nir Eyal
9. Sprint — Jake Knapp
10. The Lean Startup — Eric Ries