The optimization process and recommendations:

1. Avoid Correlated Subqueries (query line: 13): A correlated subquery is a subquery that contains a reference (column: Date) to a table that also appears in the outer query. Usually correlated queries can be rewritten with a join clause, which is the best practice. The database optimizer handles joins much better than correlated subqueries. Therefore, rephrasing the query with a join will allow the optimizer to use the most efficient execution plan for the query.
2. Avoid Subqueries (query line: 5): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.

The optimized query:

SELECT
        Minutes = SUM(r.MinutesRead),
        Pages = SUM(r.PagesRead) 
    FROM
        (SELECT
            DISTINCT r.Date,
            r.UserId,
            r.BookId,
            r.UserGroupId 
        FROM
            dbo.ReadingStatDaily r 
        WHERE
            r.Date >= @p0) r0 CROSS APPLY (SELECT
            TOP 1 r.MinutesRead,
            r.PagesRead 
        FROM
            dbo.ReadingStatDaily r 
        WHERE
            r0.Date = r.Date 
            AND r0.UserId = r.UserId 
            AND r0.UserGroupId = r.UserGroupId 
            AND r0.BookId = r.BookId 
            AND r.Date >= @p0 
        ORDER BY
            r.WordsRead DESC,
            r.PagesRead DESC) r