Gunaratana wrote Mindfulness in Plain English (which is available both as a book from the usual vendors, and also as a free pdf file). The book is considered to be something of a classic, and is written in a clear, straightforward style. In any case, The Experience of Samadhi: An In-depth Exploration of Buddhist Meditation (here on Amazon, and also as a free pdf file) by Richard Shankman contains eight “interviews with contemporary teachers”, including one with Gunaratana.
The interviews alone make Shankman’s book worth the price of admission, highlighting a wide spectrum of opinions and approaches on meditation practice.
Here’s a quote from Gunarata’s interview. He’s speaking about the complementary use of concentration and mindfulness meditation:
With concentration, you can never practice straight away without any problem. You have sleepiness, restlessness, worry, and all the hindrances. They keep bombarding your mind all the time. In those times you use mindfulness to deal with these hindrances and then to proceed with concentration. And therefore anytime you practice concentration, you have to use mindfulness to deal with problems. You cannot simply focus your mind to get you through problems. You can’t do that.