The Internet is full of discussion of the death of Benedict Anderson (see, e.g., Henry Farrell at Crooked Timber, Corey Robin at his blog and also at CT, and Robert Greene II at the USIH blog).
Although I've read (parts of) Imagined Communities and admire it and was just looking this morning at a section at the end of the 1991 revised edition (where Anderson takes off from Renan on forgetting to discuss the paradoxes attending the ways in which national histories are retrospectively rewritten to emphasize "fraternal" quarrels), I can't say the book had an enormous effect on me. Its wide influence, however, is of course undeniable.
A book that deals partly with nationalism and had a bigger impact on me is Rogers Brubaker's Citizenship and Nationhood in France and Germany (1992), not so much for its details (about 95 percent of which I've forgotten) or even for its main thesis, but rather because of the meticulous care with which it was researched, organized, and written. Today, many books published even by top-line university presses are littered with typographical and spelling errors and in some cases awkward or ungrammatical sentences; many of them are not written carefully, and they are not copy-edited or proofread competently. (Note: I'm saying "many," not "all.")
Citizenship and Nationhood, which was based on the author's dissertation, is the exact opposite: excellently written and virtually devoid of the small errors that bespeak a carelessness and haste and that are rampant in scholarly books today. I'm quite sure -- in fact, I'm positive -- that some of the arguments of Citizenship and Nationhood have been challenged since its publication, but the care that went into that book is obvious from the first page to the last. It's no surprise that the author, who has written a lot of other things since that book, has had a highly successful scholarly career.
Added later: I also liked, to some extent, Anthony Marx's Faith in Nation: Exclusionary Origins of Nationalism, which I read more recently.