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If you are interested in sup-norm and Hölder estimates, then Lunardi's book is a good start:

http://www.amazon.com/Semigroups-Regularity-Parabolic-Differential-Applications/dp/3764351721/ref=la_B001K6J69O_1_1?ie=UTF8&qid=1349596684&sr=1-1

Otherwise you should specify what type of equation are you interested in.

ADDED: Afret the comment of @Liviu:

You should not omit Krylov's books from your list: the one on Hölder spaces and the one on $L^p$ spaces.

And of course there is Evans. An excellent introduction.

ADDED: After the clarification in the question: Topping's lecture notes (there is also a book version from the London Mathematical Society) are quite nice and readable.

2 added 433 characters in body

If you are interested in sup-norm and Hölder estimates, then Lunardi's book is a good start:

http://www.amazon.com/Semigroups-Regularity-Parabolic-Differential-Applications/dp/3764351721/ref=la_B001K6J69O_1_1?ie=UTF8&qid=1349596684&sr=1-1

Otherwise you should specify what type of equation are you interested in.

ADDED: Afret the comment of @Liviu:

You should not omit Krylov's books from your list: the one on Hölder spaces and the one on $L^p$ spaces.

And of course there is Evans. An excellent introduction.

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If you are interested in sup-norm and Hölder estimates, then Lunardi's book is a good start:

http://www.amazon.com/Semigroups-Regularity-Parabolic-Differential-Applications/dp/3764351721/ref=la_B001K6J69O_1_1?ie=UTF8&qid=1349596684&sr=1-1

Otherwise you should specify what type of equation are you interested in.