"You will, I am sure, agree with me that... if page 534 only finds us in the second chapter, the length of the first one must have been really intolerable"

This quote by Arthur Conan Doyle is referring to a book that the speaker and the person they are speaking to have read. The speaker is suggesting that the very first chapter of the book was so long and tiresome that they have actually only reached page 534 and are still in the 2nd chapter. The speaker is recommending that the person they are speaking to will concur with them that the length of the first chapter was intolerable. This quote is an amusing method of revealing the speaker's opinion that the book was too long and tiresome. It is also a way of revealing the speaker's surprise that they have actually just reached the second chapter regardless of having read numerous pages. The quote is a funny method of expressing the speaker's viewpoint that the book was too long and tedious.