Unless you are planning to house an existing collection of books in your new home library you will inevitably end up with more empty shelves that you want. It is possible to fill that space with classic decorative pieces: bookends, statues, globes etc. You should always consider whether your home library lacks some essentials, and if so you can purchase more books to achieve the look you might desire.
Reference books
Encyclopedias
The times when the only way to have quick access to information was to own an entire encyclopedia are gone. Computers are happily performing most of our reference needs. This means, however, that if a paper encyclopedia is something you would like to have in your house the price can be much more affordable than in the past. Used reference volumes can be found online, as well as at yard sales. Among the most authoritative remains Encyclopedia Britannica. Keep in mind that if you have a nack for antiquarian books, the 29 volume 1911 Edition is a classic that one should be very pleased to own. There is also the first edition. Although the original one may be in short supply and not within your price range, this four-volume compendium of late 18th century knowledge has been reprinted in all its glory.
Dictionaries
The Oxford English Dictionary is arguably the best lexicographic work of all time. The full edition is cumbersome and expensive, but there is a one-volume portable edition. If you want to have an American classic, it is important to remember that Noah Webster failed to secure any sort of trademark for his dictionary. As a result, anybody can publish a Webster's dictionary. That said, if you want the best American lexicography has to offer, you should go with Merriam-Webster, preferably the 3d edition.
If you know any foreign languages (no matter how well), it is a good idea to purchase authoritative dictionaries. And even if your linguistic skills are modest, it is a general idea to have a good dictionary for every one of the most important languages: French, Spanish, German, Latin. Quotation dictionaries are also useful. Oxford publishes a good one.
Classical works
Works by classical authors are generally inexpensive. You can very quickly build a collection of hard cover volumes. If, however, you have any interest at all in what can be properly called the classics (Greek and Roman), there is an excellent series that currently features nearly all of Greek and Latin texts prior to 4th century. I am talking about the Loeb Classical Library. These small well designed and well published volumes have Greek and Latin text accompanied by facing modern English translation. If you have a few bookcases that you feel like quickly filling up with first-grade books, Loeb is the answer!
