These days just about the entire Peanuts strip has been reprinted in beautiful hardcover volumes, each encompassing two-year intervals, by Fantagraphics. But back in the day this reprint series by Holt (now part of Macmillan) was the only way to get the strips after they appeared in the daily papers.
Speak Softly and Carry a Beagle was a special book for its time: as the inaugural entry in the Peanuts Parade series of books, it was 50% thicker and printed at a significantly larger trim size than all the others up to this point. Eventually all the earlier books would be recompiled into this series as well, oddly giving this one a series number of 11, even though it was the first. And in the 90s a third of the book was gutted and it was reprinted with a new cover in a smaller format. So if you're nostalgic for these strips and want to read them 1970s-style, hunt this version down.
In terms of content, this represents just about a full year in the lives of the Peanuts gang, and its placement in the mid-70s means that the strip is at its very peak of intellect and satire before completely losing the very mild edge that it had back then. In a mere five years' time, kids would know the gang more for their commercial endorsements than for the robust psychology and commentary on real adult emotions and anxieties on which these beloved characters are based.
(Note that the Peanuts Parade series was much more successful in libraries than in the general trade, so it can be hard to find these without library bindings.)