You can store a song in your songbook to return to later by clicking the star symbol at the bottom left of the song page. The star symbol will then be highlighted red, as seen in the screenshot below, indicating that the song has been added to your songbook. To remove a song from your songbook, simply deselect the star, returning it to its unselected grey state.
You can also add and remove songs by selecting and deselecting the songs' corresponding stars in search results and on the Riffstation homepage. Songs can also be removed directly from your songbook.