This book appears a large, well-worn leather covered book. Random text titles in a myriad of languages adorn its exterior.
When you place a book flat down on a page of the book and deliberately push it down, the work is absorbed into the book. The book itself does not get physically larger, but all of the pages of the added book are added into its own chapter. By opening the book to the correct chapter, all of the remaining pages of the book map to those of the added book. A melded book can be removed from the book by tearing out the first page of the chapter - the loose page transforms back into the original. The book never runs out of pages this way.
By dropping a book into the open pages of this book, the new book is stored in extra dimensional space, The book should be of reasonable size and must be a real, readable book, not some random object with some book-like properties. To obtain the stored book, one must visualize it while shaking the Book of Holding upside down.
The big difference between Text Melding and Book Hiding is that you can read the merged works while you cannot read hidden works. It is possible for both types of storage to be used concurrently.
The book itself is very tough but not indestructible. If seriously damaged, all of its stored books will burst out of it and be scattered about. Storing powerful, sentient or otherwise magical tomes within is not advised - there could be some very unusual side effects to this. Spellbooks should be okay, but few mages would risk their precious tombs to this item.