Summer is a great time to work on your writing and move forward with your book. A lot of people are on vacation or getting time off from work, so it's a waste not to take advantage of that free time you suddenly have. You can take the time to start your new project, or to finally finish your book.
If your book is practically finished, now is the time for the final push. Your manuscript needs editing and proofing, so find the right editor that will turn it into a success, search for a publisher (or a web-to-print provider that can print your book), think about your cover design, and finish the last touch-ups of your book to make it ready to be published.
If you're still in the process of writing, take the summer time to finish your project. Set a due date to finish your book and do your best to accomplish it. Set small daily goals that you can achieve like writing for two hours, finishing the titles for each chapter, or deciding on the names of all of the characters.
Lastly, if your book is already published, use the time to think about marketing. A great marketing plan is the key to a successful book, as nobody will buy it if they don’t know it exists. Summer is a great time display your book at community events, book store signings or interviewing for the local radio or TV channel.
However you decide to spend your summer, make sure you make the best out of this season and promote your book in any way you can. Good luck!