Here is a handful of books the focus on Extensive Reading that you might find helpful.
Extensive Reading in the Second Language Classroom, by Richard R. Day and Julian Bamford
Cambridge Language Education, 1998
This is the book that initially defined “Extensive Reading” and which provided a guide towards setting up and effective program. Later articles have challenged many of the “charcteristics of extensive reading” mentioned there in ascribing some characteristics as more essential than othes, depending on the learning context.
Extensive Reading Activities for Teaching Language, by Julian Bamford and Richard R. Day
(Cambridge Handbooks for Language Teachers, 2004
This volume contains contributed articles from ER practioners on how to extend the benefits of ER when used in the classroom.
A Community of Readers: Interviews with Extensive Reading Scholars and Practitioners, by Michael McCollister (Editor), 2014, Amazon Kindle Edition
Community of Readers is a collection of interviews with experts in the field of extensive reading. These include David R. Hill, Paul Nation, Rob Waring, Marcos Benevides, Philip Prowse, Syying Lee, Jennifer Bassett, Wendy Lambert and more. The purpose of this collection is not to offer a prescribed set of rules regarding extensive reading as it to ask readers to participate in a discussion of ER theory and practice.
Extensive Reading, by Richard Day and Jennifer Bassett (2016). Into the Classroom Series, Oxford University Press, Revised Edition
Describes and explains what is meant by extensive reading and its contribution to language learning; explains what is meant by a graded reader and what makes a good graded reader; Understand how a graded readers series is written and how it supports extensive reading; Explains how you can use class readers, class libraries, and class reading circles; Includes four Case Studies to provide an insight into setting up extensive reading programmes in Jordan, Bahrain, and Japan. (From OUP promo site)