_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
	}
	
	err = db.Ping()
	if err != nil {
		db.Close()
		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
}

@avoidik

Nice catch. Although, If the function were more bulky than it was then this style would be beneficial. But as it is, wouldn't it be cleaner to just do

......
......
if err != nil {
db.Close()
return nil, err
}

....