As you can observe by a look-see at Acrobat's Link Tool dialog, there is no facility for adding a "tool tip" or an alternate text description.
Nor can such be provided upfront in an authoring file and migrated into the output PDF.
While a properly developed Tagged PDF supports providing Alternate Text description for the <Link> element this is focused to AT applications.
Typically, entered Alt Text for the <Link> element will not "display" on screen ala "tool tip" idea.
Keep in mind that the feature set associated with a PDF link annotation is not at all the same as what is associated with HTML elements.
PDF (ISO 3200-1) and HTML (any flavor or 'recommendation') are rather different creatures.
What I do is I create a word doc first, which contains the link, then save as pdf... and ta da! Links in the ebook!