In "Letter to His Son", how does Robert E. Lee describe and argue against secession?

General Lee's letter to his son is noteworthy because it describes secession as a very mournful process, in stark contrast to the jubilation that many Southerners displayed towards it.  Lee describes secession as disastrous for the country, something he terms as "no greater calamity for the country than a dissolution of the Union."  In further his claim by suggesting that secession as "anarchy," Lee suggests that secession goes against the very essence of the vision that the framers had in crafting the nation.  For Lee, secession defies the notion of the "union" that is articulated in the Preamble to the Constitution:

The framers of our Constitution never exhausted so much labor, wisdom, and forbearance in its formation, and surrounded it with so many guards and securities, if it were intended to be broken by every member of the Confederacy at will. It is intended for perpetual union, so expressed in the preamble, and for the establishment of a government (not a compact) which can only be dissolved by revolution, or by the consent of all the people in convention assembled.

For Lee, the argument against secession is rooted in the nation's founding document.  Lee cannot zealously embrace a path that he feels goes against the spirit of the nation's founding and the intent of its architects.  It is in this light where Lee's description of secession, especially in light of the destructive nature of the Civil War, is significant.

In Robert E. Lee's "Letter to His Son" (1861), what gift does his son give him and how does it relate to his feelings on secession?

Lee's son, Custis, had given his father a book on George Washington.  The gift enables Lee to delve deep into the condition of the nation in 1861 and how this would be perceived by framers of the country like Washington.  For Lee, the gift allows a sense of reflection in examining from where to where the country has gone.  The recognition of this gift links to his overall ambivalence about secession.  In the letter to his son, Lee is able to articulate the political and personal reasons why secession might not be the best approach for the South to take and for the nation to endure.  Lee states clearly that his feelings about secession are not positive ones:

But I can anticipate no greater calamity for the country than the dissolution of the Union. It would be an accumulation of all the evils we complain of, and I am willing to sacrifice everything but honor for its preservation. I hope, therefore, that all constitutional means will be exhausted before there is a resort to force. Secession is nothing but revolution.

While he agrees with his son that the "The South, in my opinion, has been aggrieved by the acts of the North," he is not ready to embrace secession as an absolute answer.  With the gift of the book on Washington's life and reflecting on his contribution to the nation as a union, Lee suggests that secession would destroy the vision that the framers like Washington has for the nation:

The framers of our Constitution never exhausted so much labor, wisdom, and forbearance in its formation, and surrounded it with so many guards and securities, if it were intended to be broken by every member of the Confederacy at will. It is intended for perpetual union, so expressed in the preamble, and for the establishment of a government (not a compact) which can only be dissolved by revolution, or by the consent of all the people in convention assembled.

It is in this light where Lee's recognition of the gift enables him the ability to reflect on how the issue of Southern secession would be perceived by the nation's architects.  Lee understands that this course of action, coupled with its dashing of the hopes of the nation's founders, will result in a state of personal sadness:  "Still, a Union that can only be maintained by swords and bayonets, and in which strife and civil war are to take the place of brotherly love and kindness, has no charm for me. I shall mourn for my country and for the welfare and progress of mankind."