Don't know about you but I love browsing through old books but there aren't many of the old type 'junk' shops about these days and the modern 'antiques' dealers know how to charge. Just a few days ago I discovered the Internet Archive where there are copies of about two million old books, all out of copyright as I don't think I have seen one dated later than the 1920s. They are all available for downloading in a few different formats which would suit many of the available ereaders.
If you have nothing else then for the Windows PC, Linux and Apple there is a free program called Calibre which can not only read many different formats of ebooks but can also make a good try at converting them to suit many ereaders. Calibre can be found HERE. Of course Adobe Reader is all you need on the PC to view the pdf version of the books.
I have tried a few of the books in the Biodiversity Heritage Library section. In this library you can find a tag cloud which speeds up the search for your choice of topic. First I downloaded a few illustrated British bird books in pdf format for transferring to my ipad but it couldn't display them properly. Don't know why as it manages other pdf documents perfectly. Next I tried the standard epub format and that seems to work OK.
If you are going to read the books on a computer then pdf versions are more accurate as they are usually scanned. Epub versions seem to have used text recognition software which is not always accurate.
Other sections available in the Internet Archive include American and Canadian Libraries, plus a Children's library - all very old books remember! You will also find access to a few electronic lending libraries which I haven't investigated as yet. Additionally the site will store freely available text material under the Creative Commons Licenses so there may be some more up to date texts to be found in some sections.
There is more to the site but I will leave that for you to explore if you are interested.
Here is a view of Adobe Reader showing the pdf download of Birds of the British Isles by John Duncan, published by Walter Scott Ltd in 1898
... and this from the pdf version of Birds of Britain by J. Lewis Bonhote, published by Adam and Charles Black in 1907
Where can you find this Aladdin's Cave? HERE
15 hours ago