Writing non-fiction has made me a lover of bibliographies. When I pick up a history book, I skip directly to the bibliography. It's a great gauge of the seriousness of the author; a threadbare bibliography is something of a red flag. My eye jumps immediately to primary sources – another measure of the books authenticity – but also for the joy of learning about sources I never knew existed. Secondary references are fun, too, and a useful research tool if I'm tackling a similar subject in my own work. Comparing bibliographies of several books devoted to the same subject will reveal the most esteemed sources and give hints of generally accepted historical "truth"
Sometimes bibliographies contain odds and ends of informantion that fall outside the demands of the larger manuscript, but are too compelling for the author to leave out. Heck. there have been bibliographies I've enjoyed more than the book they served.