The Art of UNIX Programming by ESR
Not saying this book is 100% correct and perfect, but most parts of it is still relevant and refreshingly insightful even after 20 years. I sure learned a lot about great engineering and generally how to approch and tackle difficult problems from it.
Aha! Reminds me of the good old days when I tried to minimize boot up time on my puny Ivy Bridge i5 laptop. Those days were fun!