{PDF} An Introduction to Continuum Mechanics Morton E. Gurtin (Eds.)

I am of the school that there is no “best” book on any subject matter. Rather, the serious student should look for a spanning set of texts.

Defining “best” requires a point of reference and it it clear that a beginner is looking for something different than a researcher.

In any case, let me say that if you have taken a beginning course in continuum mechanics, this is an excellent second book.
To give this book to a novice and expect them to learn the subject would be like giving Foundations of Mechanics by Marsden to someone and say “go learn classical mechanics”.

Of all the classics, I think it is best to start with Malvern. It is a good place to absorb the basics and learn index notation. It is the first book I would recommend.
Next, progress to Gurtin. The elegance and utility of direct tensor notation will then become clear. Also be aware that Gurtin is a mathematician and approaches the material from the perspective of elegance as
opposed to physical insight.