Hi you could also use NTILE.

As an example

WITH sampledata AS (

SELECT *, NTILE(5) OVER (ORDER BY Numbers DESC) N

FROM (VALUES

('A',134)

,('B',122)

,('C',88)

,('D',82)

,('E',33)

,('F',22)

,('G',18)

,('H',4)

,('I',1)

,('J',1)

) SD(Book,Numbers)

)

SELECT Book, Numbers,

CASE WHEN N = 1 THEN 'TOP 20%'

WHEN N = 5 THEN 'BOTTOM 20%'

WHEN N = 3 THEN 'MIDDLE 20%'

ELSE ''

END

FROM sampledata

--WHERE N = 3

Edit: cleaned up query a bit to make it clearer