When you authorize ADE to be signed in anonymously without an ID, it generates a unique user serial number that is destroyed as soon as you log out of that anonymous sign in. There is no way to recover this previously generated unique user serial number nor will signing back in anonymously work as a new unique number is generated. It is always recommended to sign in with an AdobeID or a VendorID account so that the book you purchase and download is always associated with your account. Unfortunately, the only thing that you can do is request another download from the bookstore which you purchased the book from, hopefully free of charge. Otherwise you'll have to purchase it outright if they won't provide it free of charge.
And that's the solution? Is the only solution ?, please is a common mistake and otherwise quite easy to make, since there is no warning that the book is not going to never to open again. Luckily, my book does not cost me much money, but there are books that slopes hundred dollars, what about people who spent that much money ?, buy another as if nothing happened ?, I'm very upset with your program and how you handle these matters, more so because there is money to everyone who wants to buy an original book and not pirated, and this is the way we reward our favor the legitimacy of the products. I have seen many people complaining about this problem, I suggest you start looking for another resolution, ask the vendor for a new download is not a solution because they refuse to do it for free.
Unfortunately, the way that Adobe designed ADE to allow anonymous signins and because of how their licensing system works, this is a byproduct of it. A book is locked to a specific UUID (Account) and when ADE is signed in anonymously the book gets locked to that UUID. If you sign out of that anonymous ADE signin, the book is lost forever as that UUID can't be retrieved again. As you say, it's a common theme in this forum for instances such as this. There is not another resolution to be had and there is no recourse from Adobe at this time. That book is permanently lost. Your best bet is to explain the situation to the bookstore you bought the book from in hopes that they might provide another download for free and to always remember to sign into ADE with an AdobeID in the future.