SQL Server 2005: TRIGGER AFTER INSERT

Hello,
I am learning SQL Server 2005.
I need to create a trigger which increments number of book's
publications:

CREATE TRIGGER InsertPublication
ON Publications
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON;
DECLARE @Num smallint
SET @Num = SELECT NumPublications FROM Books WHERE ISBN IN
(SELECT ISBN FROM inserted);
UPDATE Books
SET NumPublications = @Num + 1
WHERE ISBN IN
(SELECT ISBN FROM inserted);
END

Unfortunately I receive a message:

Incorrect syntax near the keyword 'SELECT'.

Could you explain me please how to correct the code?
I am new to SQL Server.
Thank you very much.
/RAM/