For this hybrid PC, the keyboard dock is not just an attachment to the main tablet's body.
The Transformer Book goes two steps further by fitting an extra battery and an additional hard drive into the dock.
This drive adds valuable internal storage - 320GB for the Core i5 version and 500GB for the i7 version - to the core 128GB solid-state drive found in the tablet.
Users can copy their data to either drives, but they will lose access to the hard drive when the tablet is removed from the dock.
The drawback of adding a hard drive becomes apparent during our battery test. Instead of acting as a secondary power source, the dock's internal battery is consumed by its hard drive.
But Asus has a ready-made solution. Pressing a special short-cut key brings up a settings page where you can manually disable the hard drive and ports, leaving the keyboard and touchpad unaffected.
The Transformer Book is inspired by Asus' Zenbook series of ultrabooks, from the aluminium chassis to the concentric pattern on the lid. Both tablet and dock weigh close to 1kg each, which makes for a hefty 13.3-inch laptop when combined.
The Transformer Book also feels top heavy and unbalanced. Push hard on the screen and it risks toppling over.
As a tablet, the Transformer Book's 13.3-inch touchscreen is large and unwieldy.
It is also quite reflective. Fortunately, the screen is bright and viewing angles are good. Its relatively high resolution (1,920 x 1,080 pixels) ensures that text looks sharp and crisp on the screen.
The Transformer Book was announced last year and hence misses out on Intel's latest Haswell processors.
Instead, it has an ultrabook configuration typical of the previous generation, such as an Ivy Bridge-class Core i5 CPU and 4GB of RAM.
General system performance meets expectations - it managed 4,070 in PCMark 7 - though the integrated graphics is not fast enough to run most modern games at default settings.
As a stand-alone tablet, the Book lasted 3.5 hours in our battery life test. You should get roughly the same uptime if you dock the tablet to the keyboard without disabling the hard drive.
But if you do turn off the hard drive in the dock, this increases to five hours.
Asus is selling the Core i5 Transformer Book at $1,898 and the Core i7 version at $2,098.
Its closest rival is probably the Microsoft Surface Pro, which starts at $1,328 for a similar 128GB model. Of course, you do have to pay for the Surface keyboard and other accessories to get the same features as the Transformer Book.
Adding a hard drive to the dock of the Transformer Book seems like a good idea, but it adds to the weight and affects the battery life.
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