Old but really good is John Nunns "Learn Chess Tactics". Beside the classic book form there are also interactive ebook versions.
In the PDF-preview which is part of the description in the link below you will see that the book goes through all tactical motifs (fork, skewer, deflection, etc.). Each is described
1) in general with with a view examples which are explained with a lot of text and
2) followed by a section of exercises which are not very hard.
Personally I find it useful that they are not hard because the main topic of this book is not to train your calculation skills but to detect the standard tactical patterns quickly.
http://www.gambitbooks.com/books/Learn_Chess_Tactics.html
In the PDF-preview which is part of the description in the link below you will see that the book goes through all tactical motifs (fork, skewer, deflection, etc.). Each is described
1) in general with with a view examples which are explained with a lot of text and
2) followed by a section of exercises which are not very hard.
Personally I find it useful that they are not hard because the main topic of this book is not to train your calculation skills but to detect the standard tactical patterns quickly.
http://www.gambitbooks.com/books/Learn_Chess_Tactics.html