Does Revelation 22:19 apply to the whole Biblical canon or only the Book of Revelation?

Yes we question that ourselves every day. Yet God addressed this with Moses from the begging: Deuteronomy 4:2 Do not add to what I command you and do not subtract from it, but keep the commands of the Lord your God that I give you. So we know that at least the torah and Revelation was given to us directly via God. The question has always been "what else" so Dogma and the Biblical canon have answered this for us. The the Apocrypha is still in question whether is from God or not. But Dogma will answer this soon.

The Deuterocanon, both Catholic and Orthodox, was written before the time of John's revelation, so that verse clearly does not condemn them; generally, the phrase "the book of this prophecy" seems to refer solely to the Revelation and not to any of the other writings later combined with it (some more dynamic translations have a variant of "this book of prophecy," which muddles this point).

It may also be relevant that the Revelation (also known as Apocalypse) was nearly excluded from the canon, as numerous other apocalyptic writings (including the Apocalypse of Peter) were.

One interpretation; many applications.

That is a good mantra for all students of biblical interpretation, I believe. It applies not only to verses of relevance to the practical, day-to-day issues and the steps we need to take to get our beliefs to agree with our behaviors, our attitudes to jibe with our actions, but it also applies to certain prophetic passages in the Bible.

Consider an analogy: you are viewing a mountain from the valley below, knowing there is an even greater mountain beyond the first, but because you are closer to the first mountain, the second mountain is obscured from your vision. The Ethiopian eunuch, for example, was reading the scroll of Isaiah when Apostle Philip the evangelist came alongside his chariot. When the eunuch came to the part of Isaiah 53, where it says,

"HE WAS LED AS A SHEEP TO SLAUGHTER; AND AS A LAMB BEFORE ITS SHEARER IS SILENT, SO HE DOES NOT OPEN HIS MOUTH. IN HUMILIATION HIS JUDGMENT WAS TAKEN AWAY; WHO WILL RELATE HIS GENERATION? FOR HIS LIFE IS REMOVED FROM THE EARTH" 53:7,8),

he was clearly confused by what he was reading, so he asked Philip to explain it to him, which Philip did. Starting with that passage and then moving on, we assume, to other similar prophetic passages that may have had one particular application either to the generation to which the prophecy had been given, to a soon-to-be generation, or to a far-distant generation, hundreds or thousands of years in the future (again, think of several mountain peaks, and the view of them from a valley beneath them), Philip took the eunuch on a survey of the prophecies concerning Messiah Jeshua (Jesus).

Isaiah 53, Christians believe, speaks of the humiliation and subsequent death of Messiah Jesus--that was the first mountain, but it also speaks of the exaltation of the same Messiah Jesus in the distant future when

"He will see His offspring, He will prolong His days, And the good pleasure of the LORD will proper in His hand. As a result of the anguish of His soul [i.e., the first mountain peak], He will see it and be satisfied; By His knowledge the Righteous One, My Servant, will justify the many, As He will bear their iniquities. Therefore, I will allot Him a portion with the [or a] great [multitude] [i.e., the second mountain peak], And He will divide the booty with the strong [that is the third mountain peak]"

My point in taking you step by step in my prolix answer is to suggest the following:

"The apostle John's warnings in verses 18 and 19, with verse 18 warning us not to add to the revelation and verse 19 warning us not to subtract from the revelation, have a double application. The first and primary interpretation is to John's first-century audience and the generations to follow who would read The Revelation of Jesus Christ, but a secondary application is to the yet-to-be completed canon of Scripture, which taken as a whole contains many prophecies, the most important of which concern Jesus. In Revelation 19 we read,

"I [an angel of God] am a fellow servant of yours [i.e., John] and your brethren who hold the testimony of Jesus; worship God. For the testimony of Jesus is the spirit of prophecy" (v.10).

In other words, the Old Testament, as well as the words of Jesus, some of which were available to John's generation in written form, are replete with references to Jesus, whether first-mountain references pertaining to His brief earthly sojourn of 33 years, the second-mountain references to His return to earth one day, or to third-mountain references to the establishment of His eternal kingdom:

"Nevertheless we , according to his promise, look for new heavens and a new earth, wherein dwelleth righteousness" (2 Peter 3:13 KJV).

Therefore, to add or subtract from any of those prophecies is to violate the "spirit of prophecy," which is the testimony of Jesus as it pertains to the various stages of His ongoing kingdom-work in the hearts of His children. This work has its basis in what He accomplished on their behalf through His death, burial, and resurrection, as well as what He will accomplish in the wake of His second coming in the air and His second return to Planet Earth, when Satan is finally defeated once and for all, and God's saints will live and reign with Him forever. When God's will will be done perfectly "on earth as it is in heaven."

Source: Royalty Jr., R. M. (2004). Don't Touch This Book!: Revelation 22:18-19 And the Rhetoric of Reading (in) The Apocalypse of John. Biblical Interpretation, 12(3), 282-299.

Someone has done a thesis on this topic. The author's thesis is that:

...how Revelation reads from the scriptures and how it attempts to con(trol) its audience functions as part of this polemic against other Christian and Jewish leaders. As apocalyptic-prophetic script, it subversively reinscribes the Hebrew scriptures and circumscribes its audience for the purpose of ideological condemnation.

Rev 22:18-19 is emblematic of the attack on John's opponents and the way the Apocalypse reads the Bible.

Basically, I think the Book of Revelation, that verse in particular, is a way to give authority to that book and make it sound like it has God's back-up.