The Professional electronic edition, in its classical form, comes as a personalized
high-quality PDF file that can be read, searched, annotated, and printed. The
cross-references in the text are active links: when a proof cites
"Lemma 3.1", for example, click on the "3.1" to be taken to that
lemma. For each result or proof, there are links in the margin to
take you to the results needed for this proof, or in whose proof the
current result is used.
This Professional edition is intended for personal use, and will
carry your name as its opening password. It can be used on most
computers, tablets and smartphones; if in doubt, try this sample chapter before ordering. You may keep
separate copies on all your computers or devices simultaneously, and
print as much of the book and as often as you like – including the
Alternatively, you can download a Professional edition from inside the free iOS app. It has similar annotation features as the classical PDF version: coloured pencils, markers, text fields and so on, as well as bookmarks and internal links. But it is not made for printing. However you can save your annotations to the iCloud and synchronize them between different iOS devices. You can try this out for free: the free app comes with a sample chapter that has all the annotation features of the Professional iOS edition.
The Professional edition in either format costs the same as the printed book.
Unlike the print edition, however, every Professional electronic edition includes free upgrades to all future editions of the book. In the iOS version, you can even download past editions for free. (These include some classical proofs that were later replaced by what I think are nicer proofs, but they can still be interesting to compare.)
The iOS version of the Professional edition exists in all five languages (English, German, Japanese, Russian, Chinese); the classical PDF version exists in English and German.
If you would like to print handouts for a course you are teaching
from the book, you can get your Department to obtain a licence for
that too; see here for details.