Recent days will always be past tense. An easier or less formal way to say it would be "as of late" or just "lately." Hope this helps!

"Recent days" always means the past. There is no rule about how long a period of time is "recent." You have to guess that from context. Even "recent days" is ambiguous. It probably means a few days, or a week, or a month. However, "days" can just mean "times." "In recent days" could possibly the same thing as "nowadays" or "of late." To be specific, one could say "in the last few days."

thank for your detailed explanation

A more common expression is to say, "I have recently been reading a book on death." Or, "I have been reading a book on death recently." (The position of "recently" slightly changes the meaning. In the first example, the emphasis is upon the book on death, In the second example, the speaker is emphasizing when he/she was reading it: recently. "Recently" is very common and appropriate both informally and formally. The expression "as of late" is not informal in American English but in fact more formal/literary. I can't imagine several construction workers talking with their friends and saying, "I saw an interesting TV program as of late." (This may be different in British English.) "Lately" most often occurs at the very beginning or very end of a sentence. "Lately I've been reading a book on death. "I have been reading a book on death lately." It sounds awkward to me to place it in the middle. By contrast, "recently" can be placed at the beginning, in the middle or at the end of the sentence. It's very flexible and very common. "I am available all of the time" (singular) or "I am available at different times." You can't say "recently" here because "I am" is in the present tense and "recently" refers to something that at least started in the past. However, you could say, "I have been available recently but am no longer available."