One my friend gave me the book some years ago. Since that, I recommend it as the first book for those who want to deeply understand results in continuous time Finance, but perhaps have no deep knowledge in Probability and Random Processes.
The book gives a short, but sufficient introduction to continuous time stochastic calculus (basics of Probability, sigma fields (needed to model information flow), change of measure, Brownian motion, Ito integral and the famous formula, stochastic differential equations (SDE)) and passes to the results in Finance. It gives both main approaches to pricing: as expectation with respect to a risk-neutral measure and as a solution of an SDE. Moreover, the author considers not only European contingent claims, but also American and exotic options. He gives a very good introduction to the interest rate models. Models with jumps are also included.
It is very comfortable to read the book due to the clarity of the exposition and numerous examples. The exercises at the end of each topic in the most part are computational, what is very good for those who intend to implement the theory, but perhaps is not very good for mathematicians (there is a small number of problems you can solve just in mind during your way to home). I read the book with pleasure and take many things from it for the courses I conduct at the Moscow State University. Many thanks to the author for the book!