Emms is more or less OK for the TK with 4.d4 (although his treatment of the Max Lange is outdated by now, as the author himself admitted in 1.e4,e5 dangerous weapons).
But his repertoire against 4.Ng5 is the weakest part of his book. Somewhere in this section of the forum, you will find some examples of superficial analyses.
Still we shouldnt be to harsh with criticism as the book is by now 10 years old.
More up to date and more comprehensive sources are books by
Palkovi and
Pinski. Even though Palkovi's book is slightly older I prefer it over Pinski as the latter contains more errors and is less well researched.
Free resources on the internet are the columns of
Tim Harding (Kibitzer) and
Stefan Buecker (over the Horizon) on the site
www.chesscafe.com (dig into the archives for an interesting update on the Max Lange by S.Buecker for instance)
And of course you should consult
this forum and the subscription area (for e4.e5 as well.

.
In fact subscribers can spare most of the digging into a very large number of forum posts, as the digest of the forum TK has been collected and annotated (by someone named Micawber) in a small CB-database that can be downloaded there.