Perhaps my family is unusual, but we often like to read similar books. Tonya and I frequently both read any given book that appears in the house, and Tristan both poaches books from our science fiction shelves and encourages us to read books he's received as presents, such as the and  series.
That has been one of our main problems with purchasing many ebooks in ways that couldn't be shared easily between our various devices. (To be fair, other problems are the desire to own the physical instantiation of a book we truly enjoyed, and the extremely low cost of buying large numbers of used books at the semiannual.)
A recent comment left on "" (23 June 2010) complained that there was no way to share books purchased from the iBookstore within a household, as there is with the Kindle. That struck me as wrong, so I looked into it, and I discovered that it was indeed wrong. It's easy to share books from the iBookstore within a household, even when each person has his or her own iTunes account, and the process is very much like the way the Kindle accomplishes that goal.
On the Kindle, or in any of the Kindle apps, you register a particular Amazon account, and once registered, any books you've purchased appear (they appear in the Archived Items view, so you must access them to move them to your Home view). Nothing prevents you from deregistering your account, and registering, for instance, your spouse's account. Once you do that, your spouse's books appear in the Archived Items view for you to read.
The only slight problem is that you must know your spouse's password. If you and your spouse prefer to keep your passwords secret, to prevent the other from snooping your Amazon purchases, for instance, you could easily have your spouse do the registration while hiding the password from you, download the books, and then deregister that account, so you'd be unable to use it for any purchases. (Perhaps a better example is sharing books with a child; you very well may want to transfer books to your child's Kindle device or app without sharing your password.)
When I looked into how one might share books purchased from the iBookstore, it turned out that there were multiple approaches that would work.
Shared Account -- It's worth mentioning that, just as with the Kindle, sharing a single account eliminates all confusion. If you have five or fewer computers, they can all access the same iTunes account and share the same purchased music, movies, apps, and books.
Such account sharing may not be a good idea, however. Since the Kindle uses your Amazon account, it might not be wise to share it with your spouse if you want to keep ordered birthday presents secret, or with a child who could potentially place a significant order without your permission. With iTunes, the liability is less, since you can purchase only media and apps, but it still feels wrong.
I think that's because it's like having your own library card - it's not that you necessarily wish to check out "" (be sure to read the From the Author note about it on Amazon.com), but that it's your business if you do.
Sign Out/Sign In -- For most people, Apple implemented a system with iBooks and the iBookstore that's nearly the same as Amazon's approach. From the main Library view in the iBooks app, tap the Store button, and then tap Purchases in the toolbar. The Account button shows you the current iTunes account that's signed in; tap it to bring up a dialog that lets you view the account and sign out. Tap Sign Out, and in the dialog that appears next, tap Sign In and then Use Existing Account. Provide the other account's username and password to sign in.
Once signed in with the second account, you can tap the Redownload button next to any book owned by that account; doing so downloads the title to your device for you to read, with no limitations. It's nice that you can pick and choose which books to download; you're not required to store all the books your spouse has purchased from the iBookstore. Even better, neither you nor your spouse need to fuss with the other's computer, which is likely much more trouble than simply tapping usernames and passwords into your iOS device.
There is one caveat. The next time you sync your device to iTunes, it will prompt you for your spouse's account's password, since your copy of iTunes isn't authorized for purchases from that account. You can either click Authorize and enter the password to sync the book to iTunes, or, if you don't really care, you can select the Do Not Ask Me Again checkbox and click Cancel to prevent it from bothering you on every subsequent sync. It's a one-time authorization though, and probably worth doing.
It's also worth noting that this works only with books purchased from the iBookstore. EPUB- and PDF-based ebooks that you have downloaded elsewhere - such as from the Take Control site - and loaded into iTunes manually must be loaded manually on each device (read on for basic instructions). There's no synchronization of such titles.
I don't know if there's a limit to the number of devices on which you can sign in and redownload purchased titles. I tend to suspect there isn't, but that you would run into the limitation of being able to link an iTunes account with no more than five computers if you tried to share broadly. Realistically, since your iTunes account probably has a credit card associated with it, it's unlikely that you'd want to share those credentials much beyond your immediate family.
Move EPUB Files -- As my colleagues Michael Cohen and Jeff Carlson pointed out in the comments on the initial version of this article, there's another approach that works: moving the EPUB files for purchased titles from one computer to another, and then syncing.
There are three ways to do this with music, but only two work with books. In each case, I believe you will still need to know your spouse's password.
Because Families Share -- Regardless of which method you choose, it's good to know that it is very much possible to share titles purchased from the iBookstore within a household. Personally, I'll stick with the sign-out/sign-in method, which doesn't require I fiddle with Tonya's Mac in any way.