May HaShem make up your losses and reward all parties for their good intentions under trying circumstances.
We will assume in this discussion what we do not actually know – namely, that the guests were responsible for the fire. Of course, realize that we have heard only your presentation and can say nothing conclusive, other than what we think you may ask for based on your version of the story. Your guests have every right to see things differently, and they are entitled to present their version to a halachic expert of their choice. If you and your guests receive different answers, you will then have to decide if there is a need for dispute resolution, which is very healthy when people go about it responsibly in the right spirit.
According to your understanding, the guests indirectly caused (i.e., gerama) the water damage to the books, which resulted from a course of action that was at least understandable on the part of the neighbors. In many cases of gerama, the indirect damager has a moral obligation to pay (chiyuv latzeit y’dei shamayim[1]). However, one should not unequivocally demand pay when there is only a moral obligation.[2] Therefore, before making any claims, you must determine how much you believe the guests owe in legal, not just moral, terms.
If the neighbors acted in a way that professional firefighters would have under the circumstances, the guests would be obligated to pay even for the water damage. One is not responsible only for the direct damage he causes, but even for the continuation of the process that naturally results. This is similar to the halacha that one who wounds another must pay for new medical problems that develop later as a result of those he caused.[3]
What if the spraying was uncalled for? The closest Talmudic precedent we found regarding such third-party damage is the gemara[4] regarding damage done during an attempted murder. The attempted murderer is exempt from payment due to the fact that at the time he caused the damage, he was simultaneously subject to being legally killed in order to save his would-be victim. If a third-party savior damages someone’s property during his efforts to save the victim, he is exempt due to a special Rabbinic enactment that was made so as not to discourage people from helping. But the fact that such a special enactment was necessary implies that according to standard halachic rules, the savior is considered the damager. Similarly, your neighbors appear to be the damagers regarding what the water caused, although they likely fall under the exemption of the above enactment.[5] The gemara implies that the attempted murderer, who precipitated the need for the savior’s strong action, is not a candidate for being obligated to pay. Therefore, in your case, the damagers for waterlog damage are the sprayers rather than the guests.
However, there is a different reason to obligate the guests: They were shomrim (watchmen) over the apartment and its contents. While shomrim are generally not obligated to pay for damage caused to land,[6] including houses,[7] this applies only to that which is connected to the ground. Since one can become obligated to watch objects based on implied understandings based on context,[8] there are grounds to obligate the guests for the books, which are movable.[9] If the guests’ negligence were to cause valuables to be stolen, for example, they would be obligated to pay for them, as preventing theft is within the implied responsibilities of one who “borrows a house.” Similarly, the guests are obligated for both fire and water damage to the books that their negligence caused. (The mechanism is halachically complex.[10])
One thing to be careful about when making a claim is estimating the value of the property. Halacha grants only compensation for the drop in value of the damaged property,[11] and people often do not realize that the amount they should be receiving will usually not be enough for them to replace such property with new items.[12]