Misplaced alignment tab character & error while using bibliography

Question

When I build my tex file, I get this error:

! Misplaced alignment tab character &.
l.6 \newblock Chapman &
Hall/CRC, six edition, 2003.
I can't figure out why you would want to use a tab mark
here. If you just want an ampersand, the remedy is
simple: Just type `I\&' now. But if some right brace
up above has ended a previous alignment prematurely,
you're probably due for more error messages, and you
might try typing `S' now just to see what is salvageable.

my bib file looks like this:

@book{a1,
    author    = "Shasha, D. and Zhu, Y.",
    title     = "High Performance Discovery in Time series, Techniques and Case Studies",
    publisher = "Springer",
    year      = "2004"
 }
,
@book {a2,
 author    = "Shasha, D. and Zhu, Y.",
    title     = "High Performance Discovery in Time series, Techniques and Case Studies",
    publisher = "Springer",
    year      = "2014"
}