What's the difference between form and shape?
I'm reading a philosophy book and these are used to denote different "things". I considered these words as almost synonyms.
You mention you are getting this out of a philosophy book, and I think the philosophical context is what might imply the difference. Philosophers might have a tendency to see "form" as reflecting the guiding principle of an object, or something connected with the meaning of it. "Form" used in this way carries a more or less vague linkage to the defining forces that cause a thing to be what it is. "Shape," on the other hand, is the more pedestrian, empirical identity, the recognizable structure of the thing.
This is also reflected in physics, wherein some "forms" of light might be (for example) ultraviolet or infrared, whereas a "shape" of light might be (again just for example) "a circle."
In large part, there are both approximately the same. Form is usually a little bit more precise than shape though.
As an example, "He is roundish in shape, but has an otherwise refined form".
However, it's just as likely that "form and shape" is being used as an expression, rather than suggesting the two have any innate difference.
It seems to me the words are used to indicate degree and relationship. My take on it is that form would be more essential and central to identity. Shape would refer to how that form manifests at the edges, how it is defined at the periphery. For example, you could have a form that looked almost like a star. When the edges of that form are further defined, the shape of a star emerges more clearly. Both are important in the observation and experience of the thing, but in slightly different ways.
However, philosophers can use terms in specific ways, with meanings that are different from those understood by the general public (and other philosophers). If that’s the case, you have to look to how the philosopher has set the meaning of those words (in the beginning chapters, at first mention of the words, or in other works).
The answer to this will depend greatly on the particular philosophy being presented. For example, in Aristotelian and scholastic philosophy, "form" often means "substantial form" and is sometimes synonymous with "nature". It is everything that determines what an entity is, except for the prime matter composing it. In the case of human beings, the substantial form is sometimes regarded as the same as the soul. Obviously, this is very different from mere shape. But there are many other philosophical systems, which may give "form" very different meanings. [Experts on Aristotelian and scholastic philosophy are invited to edit this answer vigorously to improve my undoubtedly simplistic description.]