Simon Haykin 
Signals and Systems, International Edition [PDF ebook] 

Haykin and Van Veen have designed Signals and Systems to be appropriate for both one- and two-semester sophomore-junior versions of the Signals and Systems course. The book’s integrated, balanced treatment of continuous- and discrete-time forms of signals and systems is both a reflection of the topics’ real roles in engineering practice and a clear, practical way of introducing the large range of topics covered by the course. Already known for its extensive example and problem sets, the Second Edition has added even more examples and problems. All chapters have been revised to improve clarity and organization.