Can someone keep something he borrowed from an owner or place that no longer exists?

Question

Let's say, while cleaning house clutter, someone discovered an old library book (borrowed, say 35 years ago) from a library that no longer exists. Can he keep the book?

Assume that there are no heirs or other take-over companies / orgs. that can be located. Heirs MAY exist, but you have no way to find out. Similar idea with the library. Perhaps, there is a parent library that took over but after a viable investigation, you couldn't discover what or where it is.

Updated based on comments

There is a discussion regarding the concept of Yi'ush, meaning if the owner has given up hope of getting his item back, it is considered "ownerless" and the finder may keep it.

To consider in your answer - Would the principles of yi'ush apply in, say, the case of a library book borrowed so long ago that by now, had the library existed, no one would recall the book was borrowed (the loan pre-dates computer databases in libraries. They were still using index card catalogs!) and since the library doesn't exist, now, technically, there is no "owner".

(Above is an example. Answers should be general to similar situations.)

Answer 1

You write "assuming that there are no heirs or other take-over companies ..."

This is quite an assumption! One cannot just make an estimation regarding the heirs. Thus there are Halachos regarding the steps to take in such an event.

If it is clear that the object is Hefker (i.e. no heir has a legitimate claim for ownership), regardless of who owned it in the past, as in your assumption, then obviously he could win it by one the actions that apply his ownership, just as everyone else can.

Answer 2

Assuming that at some point you were supposed to return the object, then when the due-date passed you were technically stealing that object.

As we see in the Rambam הלכות גזלה ואבדה - פרק שמיני, you may not keep items that you stole in your possession; be it a son who stole from his late father or one who stole from a convert who has no relatives.

It would thus seem that the correct thing to do would be צָרְכֵי רַבִּים - put it to public use. In this case that would mean donating it to a public library.

I think the Kitzur in סימן קפב - הלכות גנבה וגזלה agrees with this approach.

הַגּוֹזֵל אֶת הָרַבִּים, כְּגוֹן שֶׁהָיָה חֶנְוָנִי וּמָדַד בְּמִדָּה חֲסֵרָה אוֹ שֶׁשָּׁקַל בְּמִשְׁקָל חָסֵר וְכַדּוֹמֶה, אוֹ שֶׁהָיָה מְמֻנֶּה בַּקָּהָל וְהֵקֵל עַל קְרוֹבָיו וְהִכְבִּיד עַל אֲחֵרִים, וְכֵן מִי שֶׁנָטַל רִבִּית מֵרַבִּים, תְּשׁוּבָתוֹ קָשָה. לְפִיכָךְ יַעֲשֶׂה צָרְכֵי רַבִּים, שֶׁגַּם הַנִּגְזָלִים יֵהָנוּ מֵהֶם. וּמִכָּל מָקוֹם לָאֵלֶּה שֶׁהוּא יוֹדֵעַ שֶׁגָּזַל מֵהֶם, מְחֻיָב לְהַחֲזִיר לָהֶם, וְאֵינוֹ יוֹצֵא יְדֵי חוֹבָתוֹ בְּמַה שֶׁעָשָׂה צָרְכֵי רַבִּים‏

However, upon further research (e.g. here) it seems that as long as the borrower hasn't returned the object, he is considered a borrower and not a thief. (Assuming he uses the object for its original intent; reading in this case.) So maybe you remain a borrower forever.

Yi'ush doesn't seem to apply to borrowed items, it seems. (E.g. here.)