_tree

.
├── books
│   ├── handlers.go
│   └── models.go
├── config
│   └── db.go
└── main.go

db.go

package config

import (
	"database/sql"
	_ "github.com/lib/pq"
)

type Env struct {
	DB *sql.DB
}

func NewDB(dataSourceName string) (*sql.DB, error) {
	db, err := sql.Open("postgres", dataSourceName)
	if err != nil {
		return nil, err
	}
	if err = db.Ping(); err != nil {
		return nil, err
	}
	return db, nil
}

handlers.go

package books

import (
	"bookstore/config"
	"fmt"
	"net/http"
)

func BooksIndex(env *config.Env) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.Method != "GET" {
			http.Error(w, http.StatusText(405), 405)
			return
		}
		bks, err := AllBooks(env.DB)
		if err != nil {
			http.Error(w, http.StatusText(500), 500)
			return
		}
		for _, bk := range bks {
			fmt.Fprintf(w, "%s, %s, %s, £%.2f\n", bk.Isbn, bk.Title, bk.Author, bk.Price)
		}
	})
}

main.go

package main

import (
	"bookstore/books"
	"bookstore/config"
	"log"
	"net/http"
)

func main() {
	db, err := config.NewDB("postgres://user:pass@localhost/bookstore")
	if err != nil {
		log.Panic(err)
	}
	defer db.Close()

	env := &config.Env{DB: db}

	http.Handle("/books", books.BooksIndex(env))
	http.ListenAndServe(":3000", nil)
}

models.go

package books

import (
	"database/sql"
)

type Book struct {
	Isbn   string
	Title  string
	Author string
	Price  float32
}

func AllBooks(db *sql.DB) ([]*Book, error) {
	rows, err := db.Query("SELECT * FROM books")
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	bks := make([]*Book, 0)
	for rows.Next() {
		bk := new(Book)
		err := rows.Scan(&bk.Isbn, &bk.Title, &bk.Author, &bk.Price)
		if err != nil {
			return nil, err
		}
		bks = append(bks, bk)
	}
	if err = rows.Err(); err != nil {
		return nil, err
	}
	return bks, nil
}