There really isn't a rule as to how recent secondary sources have to be. Normally, I'd have no quarrel with using a 25-year old book if there is no other publication with the same scope that's newer. But that's heavily dependant on the topic: The closer the book was written to the time it covers, the more difficult to use now. Generally, I'd base it on reviews on JSTOR and elsewhere, that way you can find out the caveats. If possible, try to find a more recent edition of the book with a new preface, many authors offer some perspective on recent developments there.

In this case, the book probably won't instill a wrong picture of history, but I'm sure a lot of interesting facets of the communist rule have only come to light gradually after the USSR crumbled, so it might be worthwhile to look for other material after you're finished with this one. On the other hand, reading about those topics from a standpoint of 1988 historiography might be interesting in its own.

This is one of those questions where the answer is really "it depends." Recent scholarship has a lot of merits, but that does not mean that older scholarship need be consigned to the dustbin. Sometimes, older secondary works have merits and details in them that can be more valuable for getting a better picture of the past. Although the answer "it depends," is somewhat nebulous, the following set of three loose criteria might help illuminate matters more clearly.

• Historiographic Context: historical methodologies and the historical profession are not static and fluctuate over time. The questions and focuses that animate historians in one generation sometimes do not carry through to the next. For example, nineteenth-century German historians, led by the example of Leopold von Ranke, emphasized the importance of foreign policy and high politics over domestic policy in their accounts of the rise of the Prussian state, whereas contemporary historians of the rise of Prussia (eg Hans Ulrich-Wehler, Geoff Eley, etc.) tend to look at domestic and internal issues rather than high politics. One of the important things to realize when evaluating older historical scholarship is to realize categorize what historiographic genre a particular work fits into and has methodology changes significantly since it was published. Genres like diplomatic or military history tend to age better than others because the type of sources used and historical methodologies for these genres have not dramatically changed. For example, the works of F. L. Petre, Charles Oman, and F. N. Maude are still cited and used today as major works on Napoleonic battles despite being over a century old in some cases.

• Sources again, access to good primary sources is not a static constant in the historical profession. Nor does historical research necessarily proceed in a Whig-like fashion where more sources are revealed with the progression of time. In the case of Soviet-Russian studies, the Cold War made it difficult to for scholars outside the Eastern bloc to conduct extensive research, but that eased considerably with the fall of the Soviet Union when many Soviet-era archives opened up in the 1990s, but now in the era of Putin, some of the older habits of guarding access have returned. Sometimes earlier scholars had access to archives that are now restricted or destroyed. Going back to the Napoleonic examples, a good deal of Prussia's military archives in Potsdam were destroyed during the Second World War, making scholars that actually were able to consult them quite valuable. Beyond archives, sometimes older historians have advantages of writing in their particular time. Military historians from the late nineteenth and early twentieth centuries had an advantage when they visited battlefields (which is what any good military historian worth their salt should do), they did so in a time prior to mass urbanization and its associated development. In the introduction of the second volume of Derek Beales's recent biography of Joseph II, he speaks highly of Paul von Mitrofanov's 1907 biography of this Austrian emperor not only because of its use of sources, but also because he lived within a large polyglot empire (tsarist Russian) and therefore possessed an intimate appreciation of this fundamental facet of life in the eighteenth century. To properly evaluate sources and uses of them, it becomes highly contingent to develop a keen eye towards the work in question's bibliography.

• The Historian/Author this one is a bit tricky, but again it is important to understand that historians do not exist within a vacuum. Even when a historian's thesis is old hat and does not enjoy much contemporary currency, some still manage to inform scholarship. An example of this would be Jackson Turner's Frontier Thesis is quite long in the tooth (1893), yet still mentioned in contemporary works on the American West, even if it is to qualify or disagree with this long-dead historian's thesis. Professional historians often have a mentor relationship with other historians and shepherd dissertations and other research along. Sometimes, knowledge about the work of a Doktorvater can lend insight into the work of later historians. J. H. Elliott, one of the great doyens of early modern Spain, was crucial in the careers of historians like Geoffrey Parker and Richard L. Kagan. Here's where one of the tricks of a good historian is to pay very close attention to the acknowledgements and citations which can allow a reader to reconstruct an academic genealogy.

But these are just three approaches to older works of secondary sources conveyed in a very long format.