My add to my understanding of cider vs. cider meaning:
The original origin of the word meant "strong drink". Some take that to mean fermented, and by some I mean most of the world except the US and Canada. Here in the US, cider meant more of strong as in a stronger taste as opposed to the clear filtered flavored water sold as apple juice. Even in the US, that was not really the case before prohibition, when cider was common alcoholic drink and applejack was a common hard spirit, but we hijacked the common meaning of the word and changed its usage at that time. And even here, that meaning is deteriorating as you can often now find products that are filtered apple juices sold as cider.
Even within fermented product, you will have wide variance. In the US, there are tax implications for going over certain percentages of alcohol, so the hard ciders tend to be limited. Also in the US traditionally common culinary apples were used to make hard ciders. That is not the case most other places, with some countries required that only specific cultivars are used. These apples often are ones that are so acidic or astringent that most would find them inedible raw. They are grown strictly for cider. The quality of the fermented product is anywhere from a cheap back shelf beer to a high quality sparkling wine depending of the method used and the qualities of the apples. In the US without the strict naming and ingredient laws of some countries, France coming to mind immediately, it even allows for things like ales made from fermented grains to be flavored with a little bit of apple juice or even artificial apple flavor, and then sold as a cider. As a fan of quality ciders, including attempting to get my own orchard with old French, American, English and Irish varieties going, I am not a fan of the entire image of the product being cheapened by this being permitted.
The book authors are doing you a disservice by not recognizing the range of what the word cider can signify. I can mostly only back up the general statement of others to go by where the author is and their probable audience. If from France they would likely mean a more wine like product. In the UK, a pub style fermented drink. From the US, an unfiltered fresh juice with particulate and usually a darker brown color. It is a careless, but common and unfortunate type mistake by food/drink authors, such as US authors using wet measure for dry ingredients and not differentiating between spoon sizes because they assume a US audience. In this case, it is especially important to differentiate though because you are talking about a product that might be flat, lightly carbonated, heavily carbonated, fresh juice, light fermentation, or up well over 10% alcohol. Those all cook, taste, and mix very differently and potentially violate religious and cultural rules.
I personally tried for years to use the French spelling of cidre to mean hard cider to differentiate, but it just confused people more in the US. The few that understood somewhat thought I meant they needed to go and find a good French cidre.