Do you have to shake out sefarim before Pesach, if you are not using them during Pesach and sealing the bookcase with a plastic sheet? Second question: Do you have to check boxes of toys if you are similarly locking the toy boxes away during Pesach?
Most poskim rule that sefarim do not need to be checked for crumbs before Pesach, because there is no chance that a person will come to eat such crumbs (see Or Letzion, Orach Chaim 32). Indeed, many poskim rule that for all crumbs that are less than a kezayis big there is no obligation of bedikas chametz, even if together they can reach the volume of a kezayis (see Peri Chadash 442:78; Shulchan Aruch Harav 442, KA 18).
Quoting from Maharil, Magen Avraham (460:2) explains that we are concerned for pieces of dough, which can merge together, but not for crumbs, which cannot merge together to form the volume of a kezayis. The common custom, however, is to check even for smaller crumbs, but this would not obligate checking one’s sefarim.
However, one should not bring a sefer that has not been carefully checked for crumbs to the table on Pesach, for fear that a crumb will fall into the food.
Chazon Ish (Orach Chaim 167:13, 18) is stringent concerning the checking of books, and rules that one must check the books for crumbs–because of the concern that one might come to eat the crumbs. Mishnah Halachos (7:64) also writes that one must check books, adding that this has been the custom for generations long (this probably depends on one’s family background), though Rav Moshe Sternbuch (Teshuvos Vehanhagos 1:298; Moadim Uzemanim, Haggadah) writes that there is no such custom, and expresses wonder at the ruling.
However, Chazon Ish concedes that there would be no obligation to do so if a mechitzah — a partition — is made to separate from the books, thereby preventing the chance that a person will come to eat the crumbs.
Therefore, if you are sealing the bookcase there is certainly no obligation to check the books.
For boxes of toys, one should make a basic check to ensure that there are no large pieces of chametz, such as slices of bread, wafers, and so on, which children are liable to leave behind. Beyond this, one can lock them away, and there would not be an obligation to check each toy for minute crumbs.