Apple mobile-device users will be able to download the revamped operating system iOS 4 today, free, gaining a sneaky peek at some of the iPhone 4’s new features, most notably iBooks and multitasking.

iPhone 3G and 3GS users will be able to read books on their mobile plus browse and buy titles from the Apple store using the iBooks application, available to download from, well, the App store. iPad owners will also be able to sync their existing iBooks library plus add PDF files to it so they can carry on reading a book they put down on the iPad from the iPhone.

A handy new feature will also allow iPhone 3G and 3GS owners to organise apps into folders with click-and-drag simplicity. Users will be able to access their favorites and manage up to 2,160 apps with ease. The only drawback is there will be a limit of 12 applications per folder, but iOS 4 will also allow users to send apps as gifts to friends.

Owners of the iPhone 3GS will also be able to utilise the eagerly anticipated multitasking feature. It will allow users to run their favourite third-party apps and switch between them without slowing down the performance of the main app they are using or, just as importantly, draining precious battery life unnecessarily.

While this will make life considerably easier, developers will need to update their apps before users can take full advantage of multitasking.

The new operating system also includes improvements to email such as a unified inbox for multiple email accounts, photos and the iPod app. It even allows the aesthetically-inclined to set wallpaper for the home screen as well as the lock screen, which was not possible before.

Predictive typing on the iPhone has not previously corrected spellings until iOS 4. Misspelled words will be underlined in red and users will be able to tap to display correction suggestions in Mail and Notes.

Sadly, though, owners of the original iPhone and iPod touch will miss out on iOS 4 due to a lack of computing power, and iPhone 3G will not be able to support multitasking. Unfortunately, while iOS 4 is sure to pep up the iPhone 3G and 3GS, it will not be able to shoot HD video like the high-resolution iPhone 4, which hit the online shops on Thursday and notched up 600,000 pre-orders in just one day.

Want the new iOS 4? It's available via iTunes update right now.