There is no right or wrong answer here: Some people swear by single opt-in, while others much prefer double opt-in. There are pros and cons for each.
1) You will get more subscribers.
2) There's no chance that your subscribers will get "lost" during the process of confirming their email address
1) You will get a lot of people who will enter junk email addresses to get your book. These are not "real" email addresses, so these people will never get your subsequent emails.
2) But you will still have to pay to mail these people. You have no idea which emails are "junk" and which are real, so you will just email them all. Most autoresponders charge you based on how many subscribers you have, so you might end up paying a lot more.
3) Your subscriber count will be larger, but your "engagement" statistics will be worse. You won't get as many opens or clicks as someone with a double opt-in list (because you'll have a lot of junk emails on your list!)
4) There is a chance that someone might submit a "spam trap" email address which would mean that your emails are flagged as spam.
1) You know that each of the email addresses on your list belongs to a real person
2) You're not paying to mail to junk email addresses
3) There's less chance you'll be accused of sending unsolicited mail (spamming), since all subscribers will have had to confirm their email address first
4) Your list will be more responsive
1) You will lose a lot of subscribers by asking them to confirm their email address. (But these "subscribers" are typically those junk email addresses -- not real people anyway.)
2) You will lose some "real" people during the step where they need to check their email and click the link. Some people will not receive the email (or it might go into their spam folder).
3) It's an extra step for your subscribers to take, and not quite as streamlined
We generally recommend double opt-in because it ends up being safer and costs less, but you should weigh these pros and cons and decide for yourself.