What 1840to1940 meant was not that stamps would be missing because they were issued later than your supplements. He meant that the Harris Statesman and any two volume album covering from say, 1840-1990, as yours does, with supplements, even so has to select from among the hundreds of thousands of stamps issued. Up to about 1940, about 85,000 stamps were issued worldwide. (Many thousands of more variants on those 85,000.)
Today, as I recall, the total number of worldwide stamps issued since 1840 is about 600,000. Your album with supplements to 1991 might have what, 50,000 spaces? 75,000? Therefore it has to be selective. Of course it will exclude a lot more of the expensive early stamps, percentage-wise, than it excludes from 1940 to 1990. But it will exclude stamps from 1940-1990 as well. Whether the ones you can't find spaces for were really issued after 1991 or were issued before 1991 but were excluded from your album you can tell only by consulting an up-to-date catalogue. You'll find a starting point at
www.stampworld.com/en/ though to me it's clumsier to use than printed Scott Standard Postage Stamp catalogues (but they are expensive--6 volumes, about $50 each even for ca. 2008 editions on
www.ebay.com/b/260/" rel="nofollow">ebay).
Sometimes, of course, you can tell because the stamp carries a date of an event it's commemorating or carries some other marker of when it was issued. Or has a cancellation date that is legible.
So my suggestion would be not to buy any supplements for now but to get oriented, see how many of your stamps actually are post-1991 (keep them in envelopes or perhaps acquire a stockbook or two). The Harris Statesman is a good level of selectivity for starting out with worldwide collecting. If you decide to specialize in certain countries, you can acquire individual "complete" albums for them (Scott Speciality albums or Bill Steiner pdfs from
www.stampalbums.com). You may decide that 1991 is a good cut-off point--it's before most of the newly liberated Eastern European countries started up again with stamp issuing, often in high volume. On the other hand, perhaps it's exactly those newly liberated countries that you discover you are fascinated by. IF so, then you'll want to buy post-1991 Harris Statesman supplements. But give yourself time to decide which you prefer.
You wouldn't really want a comprehensive set of worldwide albums to 2014 (quite apart from how expensive and bulky it would be)--the 5,000 or 10,000 stamps you might assemble in a few years would seem lost in a sea of empty spaces. So Harris Statesman to 1991 is a good way to start and then see if you want to go past 1991 or not.
For now, you can sort stamps using web resources, put the ones before 1991 into your Statesman albums and only buy print catalogues and supplements at the point you know better what you want to include and exclude.
And have fun.