From the main (simple) search screen, type in a keyword or phrase (using quotation marks) to locate all books in Google Books where that word or phrase appears. e.g., type the keyword "Gilead" to find Marilynne Robinson's 2004 book. Caveat: The vast majority books are not available, e.g., Richard Dawkins, The God Delusion (2006).
When (and if) you locate the book you think you want, you have a number of options: 1) link to its record at an online book retailer, 2) locate a local library that holds the book, and 3) "search in this book" for keywords. E.g., search Gilead for the word "baptism." Caveat: The full-text search option is not exhaustive and complete accurate, e.g., search Thomas Oden's The Rebirth of Orthodoxy for the word "Christian" then browse to page 51 and see where this instance of the word does not appear in the results list. Also consider plural forms. Caveat: The list of libraries holding an item are often not accurate, so double check the catalog of your nearest library, e.g., Pelikan's Credo (2003) is held at Pitts Library, but under a different record, viz., Creeds & Confessions of Faith in the Christian Tradition.
Determine how much of the book is available for viewing. The four different views
of a book are listed here. If the book is in the public domain, it may be available to download as a PDF. e.g., Since it was published prior to 1923 and scanned into Google Books, you can read the full Latin preface in George Eliot’s 1856 translation of D.F. Strauss’ Life of Jesus. Keyword search for strauss, life, jesus, eliot. Then browse backward to page 1.
To perform more specific search (by author, title, ISBN, etc) use the "advanced book search" option.