What happens if I have a Pulmonic Sliver, a Dregscape Sliver, and several Slivers on the board, and one of my Slivers dies? Do I get to return it back to my library?
Usually, if a permanent has "If this permanent would be put into a graveyard, you may put it on top of its owner's library instead" and would go to a graveyard, its controller (C.R. 109.5) may choose to put it on top of its owner's library instead. If they choose not to, then it goes to the graveyard as normal.
However, if that permanent returned to the battlefield due to unearth and would leave the battlefield, it is exiled rather than going anywhere else, including to a player's library or graveyard (C.R. 702.84a).
(Note that Dregscape Sliver grants an ability to certain cards in "your graveyard" and doesn't grant any abilities to any permanents, not even, say, "If this permanent would leave the battlefield, exile it instead of putting it anywhere else". Also, Pulmonic Sliver's first and second ability affect all "Sliver creatures" or all "Slivers", respectively, no matter who controls them.)
However, if that permanent returned to the battlefield due to unearth and would leave the battlefield, it is exiled rather than going anywhere else, including to a player's library or graveyard (C.R. 702.84a).
(Note that Dregscape Sliver grants an ability to certain cards in "your graveyard" and doesn't grant any abilities to any permanents, not even, say, "If this permanent would leave the battlefield, exile it instead of putting it anywhere else". Also, Pulmonic Sliver's first and second ability affect all "Sliver creatures" or all "Slivers", respectively, no matter who controls them.)