According to the Pauling Scale, compounds with electronegativity difference of 1.7 or above is considered as ionic compound; compounds with electronegativity difference between 1.7 and 0.4 is considered as polar covalent compounds, those below 0.4 or 0 is non-polar covalent compounds. When I check the electronegativity difference between hydrogen and lithium, it is 1.1 which is between 1.7 and 0.4, according to the Pauling Scale, LiH should be polar covalent compound, however, it is told that LiH is in giant ionic structure and the bonding is 'appreciably ionic'(while 1.1 is not just a bit smaller than 1.7 in the scale) in the text book. The same situation is found in case of NaH.

I would like to know what makes such result(and also what does the word 'appreciably ionic' mean[how ionic it is?]).

The rules are basically rules of thumb and I believe the Pauling scale is not the only electronegativity scale used. Most chemists have an idea of electronegativity difference but are rarely dogmatic in applying the differences quantitatively.

Appreciably ionic means more or less fully ionic with the cation polarising the electron density of the anion to some extent