What's the difference between releasing hormones and tropic hormones?
The releasing hormones could also be considered tropic hormones, and indeed they fit the definition as you noticed, but aren't usually named as such.

The special thing about the ones your textbook is specifically highlighting as releasing hormones is that they communicate from the hypothalamus to the pituitary through the hypophyseal portal system, vessels that travel directly from the hypothalamus to the anterior pituitary.

Therefore, they aren't really released into the broader circulation (they can get there, but they'd be in too low a concentration). They are easily identified in a list of hormone because they all share a naming convention where they are called "____-releasing hormone" and their primary function is to cause the pituitary to release whatever is in the "_____".

In terms of overall neuroendocrine function, the "releasing" hormones all cause release of tropic hormones, which in turn cause release of hormones that affect other tissues. 

Therefore, the sequence is releasing -> tropic -> non-tropic. They could have been named something else, like "primary tropic" and "secondary tropic" but this terminology keeps them separate as "tropic hormones that cause release of other tropic hormones."