"Sealing a scroll was a common and important practice in Biblical times. The wills of both Emperor Vespasian and Caesar Augustus were secured with seven seals.
"For such a document, a scribe would procure a long roll of parchment and begin writing. After a period of writing he would stop, roll the parchment enough to cover his words and seal the scroll at that point with wax. Then he would resume writing, stop again, roll the scroll, and add another seal. By the time he was finished, he would have sealed the scroll seven times. The scrolls would be read a section at a time, after each seal was opened.
"Why was this process used? Evidently it was to prevent unauthorized persons from tampering with the scroll or reading and revealing its contents. Only a "worthy" person -- that is, someone with proper authority -- could have legal access to the scroll's message.
"When a Jewish family was required to forfeit its land and possessions through some distress, the property could not be permanently taken from them. Their losses were listed in a scroll and sealed seven times, then the conditions necessary to purchase back the land and possessions were written on the outside of the scroll. When a qualified redeemer could be found to meet the requirements of reclamation, the one to whom the property had been forfeited was obligated to return the possessions to the original owner."
(There's A New World Coming, Hal Lindsey)
The Lamb, more specifically the Passover Lamb, is the symbol of Christ as the Sin-bearer of the world (John 1:29). Only the Lamb is worthy to loosen the seals of the book.
Just as a side note: In contrast to the Lamb is Satan's typical perverted imitation of Christ in the role of the false prophet who is "another beast" [i.e. in addition to the anti-Christ] who has "two horns like a lamb," but speaks "as a dragon" (Rev 13:11).
Note the description of what John saw. He clearly described a book with seven seals. An often overlooked point is remembering that John saw seven seals. Since he saw them, they must have been on the outside of the scroll, not on the inside of the document as some suggest.
Some believe that the first seal has to be broken which opens the scroll until the second seal is reached and then it too is opened, etc. This is not the case with this particular scroll in my opinion. The seals are not hidden from view. With the seals being on the outside then, all seven must be opened before the contents of the book are revealed.
Robert van Kampen owns one of the world's most extensive private collections of biblical manuscripts, with many dating back to the second century. He writes in his book, "The Sign," that of the many scrolls in his possession not one has a seal on the inside. When sealed, the scrolls are all done so from the outside. When one seal is present, it was usually placed there by the author of the scroll. When more than one seal is present it indicates a series or set of conditions that must be met before the scroll can actually be opened. He also cites that the Shrine of the Book located on the grounds of the Israel Museum in Jerusalem has many scrolls that have multiple seals placed on them, yet not one scroll has a seal on the inside.
(It has also been noted that Roman wills and legal contracts were documents that often had seven seals on them like the one John describes in Revelation.
What is the scroll or book that John saw? The closest thing we have to comparison is found in Ezekiel:
As in Revelation, the scroll here has writing on both sides. Apparently, scrolls of this nature were extremely rare in New Testament times as typically the writing was done on one side only. The writing side, called the "recto," was the side in which the fibers of the document ran horizontally thus making writing easier. The reverse side, the "verso" was usually for the title or address of the document, and was only used when there was inadequate space on the recto. In other words, a document with writing on both sides indicates that the author had much to say. Documents with writing on both sides are so rare in fact that there is even a technical name for it, an opisthograph.
As we see in Ezekiel, the scroll contained "lamentations, and mourning, and woe." This is easily comparable to the scroll of Revelation, in my opinion. The obvious difference between the two scrolls, however, is that the one in Revelation is sealed. We know that only the proper persons or officials can open a sealed document, and in the case of multiple seals, certain conditions must be met first. It may be significant to remember that Roman wills were sealed in this fashion and thus the document could not be opened until the death of the person whose will it was could be attested. Interesting in view of the fact that John describes the Lamb as having been slain (Rev 5:6,9).
So we see by Christ's sacrificial death that He is worthy to loose the seals. In addition, He also has the authority to open the book as we see in the dual imagery of Christ as the Lion of the tribe of Judah, the root of David. Here we have Christ's Kingship displayed. His role of Judge is undisputed.