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.