First of all: The actual book House of Leaves itself, although ostensibly written by Mark Z. Danielewski (whose name only appears on the book's cover), consists in-universe of material written by Zampanò under the title The Navidson Record, with introduction and notes by Johnny Truant (both characters in the book). The book contains notes by Johnny Truant on how he catalogued and compiled Zampanò's notes after his death. This is made clear on the book's title page and colophon, which does not carry Danielewski's name. So the book itself is, from the title page, metafictional.
The book's title is also mentioned in the aforementioned "Exploration #5" (on page 465 in the hardcover edition), which is part of The Navidson Record written by Zampanò:
As Navidson indicates on the recorder, he is slowly becoming more and more disoriented. He suffers from surges of nausea, “like I’ve got a bad case of the spins.” Questions plague him. Is he floating, falling, or rising? Is he right side up, upside down or on his side? Eventually, however, the spins stop and Navidson accepts that the questions are sadly irrelevant.
Taking a tiny sip of water and burying himself deeper in his sleeping bag, he turns his attention to the last possible activity, the only book in his possession: House of Leaves.
“But all I have for light is one book of matches and the duration of each ma—” (for whatever reason the tape cuts off here).
In the passage immediately following "Exploration #5", it becomes clear that this is indeed the same book. Zampanò explains how Navidson could have read the book in the dark by burning the pages he had read to make reading light for the new ones. He mentions that the book had 736 pages, and that some of them took longer to read than others because the text on them was arranged in peculiar ways – both of these facts hold true about the actual book!
Either Johnny Truant secretly re-wrote parts of The Navidson Record to include references to his completed encompassing work House of Leaves; Johnny named his book after this book-within-a-book and padded his notes to make the final page count fit; or Zampanò actually originally wrote about Navidson reading a book which did not at that point exist yet. Which case is true is up to the reader to decide. (As well as, of course, whether or not Navidson actually existed and did read any book at all while exploring the supernatural house.)
Navidson's copy of the book is not mentioned anywhere else in the text (thanks to @Valorum for confirming that, and for providing the relevant quotations in his answer), but Johnny encounters another copy of it when he attends a concert where the band plays a song with lyrics that reference it, and the band members show him their copy (which is apparently one of several):
“Take a look for yourself,” he said, handing me a big brick of tattered paper. “But be careful,” he added in a conspiratorial whisper. “It’ll change your life.”
House of Leaves
by Zampanò
with introduction and
notes by Johnny Truant
