One does not have to read a lot of books on any subject, if one reads good books on the subject. It was the wisest of men who said, "Of the making of many books there is no end" (Eccles 12.12). It is simply impossible to read every book published on many subjects! So the answer is to read good books. To give an example. In her latest brick, Gail Riplinger has a (thoroughly unnecessary) section on the Knights Templar. When I criticised it, Riplinger's defender accused me of having made a special study of the Templar. The accusation is false - I had merely given a lecture on the Da Vinci Code, and read two books on the Knights Templar to do so, both popular books, not academic volumes. In contrast Riplinger's section on the Templar quotes more than a dozen works - all of them completely worthless! Why? Because they are books of fables and myths! (The reader is directed to Riplinger, Hazardous Materials [A.V. Publications, 2009] Pp. 843-851). They include the books underlying the Da Vinci Code, works by men described by leading British academic historian Prof. John Charmley as "Fabulists" (to come clean, he's my Dad, and it was a private conversation. The full quotation was "They're not historians, they're fabulists"). Therefore before reading a book one has to consider several points. First of all, origin. Who wrote it, and what else have they written? What qualifications do they have for writing it? If there is a brief description of the author on or in the book, what does it say? Then look at the bibliography, if there is one. Who are they citing? A large number of self-references and references to self-published works ought to set the alarm-bells ringing. Some authors are always worth reading, such as D.A. Carson and John Calvin.
Time is limited, so ask the question, is this the sort of book I'm looking for? If it's for leisure reading, the style is most important. Some academic writers have a terrible written style and should be avoided as leisure reading. So don't get Martin Gilbert's volumes on Churchill, get a smaller biography. Reading for serious information, make sure that the author has actually researched the book, and always be aware that where a passing reference is made to a person or event in a book not specifically dealing with that person or event, the author may well be in error. For example, John Pollock's description of Spurgeon in Moody Without Sankey is utterly inaccurate, but then it's not a biography of Spurgeon!
Particularly in terms of history and biography, there are volumes that are basically 'fluff', popular-level book relying on other popular level books. Avoid them like the plague they are. They will not help you at all. On the other hand, some classics ought to be read by everyone. In theology, where there is an early Cunningham Lectures volume on a work, it is usually worth reading ('Early' referring roughly to the preiod before 1900). This series produced Buchanan's standard work on Justification, Smeaton's on the Holy Spirit, and A.B. Bruce on The Humiliation of Christ. If you read the best, you will not have to worry so much about the rest.
Read more than one era. Old books are always worthwhile if they are good. But also modern writers are building on what has gone before. In terms of history it is most important to remember that historical research is always going on. Older books may be based on mistakes committed in the past that have now been corrected. And remember, a popular-level writer may well simply be repeating the mistakes of others!
And read well-written books. Reading good English will help you to write good English. Conversely, reading bad English will have a detrimental effect on your English style!