From my memory of Computing A Level the textbook and availability of resources are poor. In preparation for the COMP3 exam I created a document of revision notes which covered all of the content required for the specification. This includes areas such as Hashing which are not clearly explained in the book.
These revision notes can be accessed here.
Furthermore I found the notes available from multiwingspan computing exceptionally useful, these can be found here.
Please note that these revision notes are written for the previous specification, however there is a significant amount of overlap.