_tree

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

db.go

package config

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

var DB *sql.DB

func InitDB(dataSourceName string) {
	var err error
	DB, err = sql.Open("postgres", dataSourceName)
	if err != nil {
		log.Panic(err)
	}

	if err = DB.Ping(); err != nil {
		log.Panic(err)
	}
}

handlers.go

package books

import (
	"fmt"
	"net/http"
)

func BooksIndex(w http.ResponseWriter, r *http.Request) {
	if r.Method != "GET" {
		http.Error(w, http.StatusText(405), 405)
		return
	}
	bks, err := AllBooks()
	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"
	"net/http"
)

func main() {
	config.InitDB("postgres://user:pass@localhost/bookstore")

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

models.go

package books

import "bookstore/config"

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

func AllBooks() ([]*Book, error) {
	rows, err := config.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
}

@filinvadim import of a package IS DI. And it lets your application correctly utilize Go's composition principles. See recent Gophercon talk by Uber.