Answer: The
Gemara (Shabbos 35a) cites a machlokes as to how heavy an item needs to
be in order for it to be rendered muktza. Tosafos, however, demonstrate
that the halacha does not follow this Gemara as elsewhere (ibid. 45b;
Eruvin 102a), the Gemara allows moving large items on Shabbos. Thus, the Shulchan
Aruch (308:2) writes that an item does not become muktze based on its
size or status. The Mishna Berura (308:9) adds that this applies even if it
takes a few people to lift the item.
The Mishna Berura (308:8)
and R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 20:22) write, however,
that if one would normally be hesitant about moving something because they are
concerned about it getting ruined, then it is considered to be muktza
machmas chisaron kis (valuable items which cannot be moved).
The Gemara (Shabbos 138a) teaches that there are certain acts that are
prohibited miderabanan on Shabbos because they are uvdin dechol,
mundane, weekday activities. R’
Ephraim Greenblatt (Rivevos Ephraim 1:222:4) quotes R’ Chaim Biberfeld (Menucha
Nechona 4), who writes that moving one’s furniture around on Shabbos is
considered uvdin dechol.
In conclusion, while it
is preferable to move one’s furniture before Shabbos, it would be permitted to
move a regular bookshelf on Shabbos providing that it isn’t an expensive one
that one is concerned that it shouldn’t get damaged.