We are going to dip our toe into Martin Heidegger's seminal work, "Being and Time". First published in 1927, "Being and Time" is considered one of the most important philosophy books of the 20th century. It is available for purchase in paperback for less than $20 and as an ebook for $10. Used copies should be plentiful and cheap. Best of all, it can be found as a PDF online for free. There are also numerous commentaries and guides available. Some selected websites are listed below. This book is neither short nor easy, so we will read and discuss the introduction to see how far we want to delve into it. Of course, that doesn't mean that being short and easy is necessarily bad. As a matter of fact, we have some members who fit that description, and they are really nice people.
PDF of translation by Joan Stambaugh (1996)
PDF of translation by Macquarrie and Robinson (1962)
Note: These two translations seem to be the only ones readily available in English. The latter is the “standard” in the sense that the English words chosen as translations of Heidegger's German have become the accepted English terms. The former is more recent and has been updated, but you will have to pay for the updated version.
Being and Time Wikipedia article
Stanford Encyclopedia of Philosophy's article on Heidegger
The Internet Encyclopedia of Philosophy's article on Heidgegger
Glossary of terms (every philosophy book needs one of these)
Summary of a reader's guide (This is an interesting website which contains diagrams and more term definitions and looks like it may be helpful.)